Vampires

Vampires are a high-level form of corporeal undead that are somewhat widely known by reputation, but extremely rare.
According to Van Richten's Guide to Vampirism, the origins of vampires are unknown, though there are rumours of their creation through dark magic and deals with dark gods and other dark powers. Given the evidence that Strahd von Zarovich attained his immortality through a deal with the Dark Powers, there would seem to be some truth to these rumours.
Though there are rumours there are certain marks of a vampire that make them easy to pick out — red eyes, perpetually visible fangs and claws, and pointed ears regardless of the vampire's original race — in truth, a vampire will look just like an average humanoid until the moment they strike, their fangs and claws seeming to sprout out of nowhere. Again, according to van Richten, there are subtler clues to look for when you suspect you may be in the company of a vampire: their only turning up at night, or under cloudy skies; their not entering a mortal's residence until explicitly invited; a seeming lack of appetite for food and drink; an aversion to natural sources of running water such as rivers and streams.
There is a legend that a vampire's reflection cannot be seen, and Strahd's having a room full of mirrors in Castle Ravenloft would seem to suggest some truth to that legend, though it's equally possible he was just feeling angsty about his receding hairline.

Vampire Spawn

Most people who think they've met a vampire have, in truth, encountered a vampire spawn — a moderate to high-level corporeal undead created when a true vampire drains the life from a mortal humanoid, then buries them in the ground to rise again the following night as their thrall.
So long as a vampire spawn's creator lives, they are literally incapable of disobeying their master. The party learned this firsthand while travelling with Doru, who seemed for all intents and purposes to be done with Strahd and willing to help the party, but who found himself forced to fight at Strahd's side when he came to claim Ireena Kolyana.
Upon the death of a vampire spawn's master, they are freed from the compulsion to obey, but do not necessarily become what most would consider a true vampire. The only guaranteed way a vampire spawn can become a true vampire is if their master sees fit to share their blood with spawn and elevate them to that level, which is almost unheard of. Otherwise, a freed vampire spawn may attempt to survive on its own long enough to ascend to the powers of a true vampire — or seek out the same sorts of deals that create some true vampires in the first place — but given their thirst for blood and the relative ease with which an average adventuring party or monster hunter can dispose of one, such ascensions are rare.
Vampire spawn have less powerful versions of a true vampire's claw and bite attacks, and the full force of a vampire's ability to walk on vertical surfaces and even upside-down, as the party learned when Doru dropped from the ceiling onto Osrin as a hello. They also have powers of regeneration, though these powers are not as strong as that of a true vampire. Like their masters, they are vulnerable to sunlight, radiant damage, and running water, and cannot enter someone's home unless expressly invited.
Also like a true vampire, vampire spawn can retain many of the skills and abilities of their previous mortal existences, making the acquisition of spawn with exceptional martial or arcane prowess a particular prize. However, those who draw their power from divine sources, such as paladins and clerics, may find themselves bereft of powers if they serve a master who loathes undead, or whose alignment is at odds with the actions they're asked to undertake by their master.
The party has had several encounters with vampire spawn during their time in Barovia, starting with Doru right through their most recent battle atop tuama beannaithe, where Calorfin Galamin and Imithren Orren were killed.

Known Vampire Spawn

Big Chin

Given his moniker by Sylvain, Big Chin was the one to initially abscond with the bones of Saint Andral once they were taken from Sylvain. He was taken down by Ri-An, but later got up and was one of the spawn to ultimately escape with the bones after a chase with Sylvain.

Blondie Scootcher

A snooty-looking but beautiful blonde woman who ended up trapped in a Wall of Force with Ri-An, Ayduin, and Osrin during the Feast of St. Andral's. Was ordered to sit and not bite by Strahd at Ireena's request; proceeded to evade several attempted attacks from our three heroes despite sitting on a coffin the entire time. When Sylvain pointed out she could, technically, scootch to freedom, she promptly took his advice. She later fought with van Richten, but escaped largely due to Big Chin and Imithren coming to her aid.

Calorfin (deceased)

Once a Reformer in Vallaki and a cleric of Vandria Gilmadrith, Calorfin was a member of the first adventuring party to defeat Strahd following his ascension. She was, as such, one of the first to realize the true extent of his immortality. As the mists descended around Barovia again, Calorfin came to the conclusion that the only way to truly help Barovia was to teach its people not to rebel against Strahd and anger him. She survived for over 300 years, before being captured and turned by Strahd late in 735. She was killed by Osrin at tuama beannaithe.

by sarahlis

Doru

Our party's first vampire spawn acquaintance, Doru was about 17 when he followed Azuth and his fellow Barovian villagers in a march on Castle Ravenloft. The only one to survive the attack, he was then groomed and kept as a plaything by Strahd, agreeing to become one of his spawn without any idea of the servitude it would entail, as he was smitten with Strahd, the first member of his preferred gender he'd been able to be openly romantic with. Strahd kept Doru for a few years before tiring of him after noticing the presence of Ireena Kolyana. He told Doru to go back to his father and Doru, believing he was acting of his own free will, hoping his father would take him back and comfort him in his heartbreak, did so. Unfortunately, his priest father did not take Doru's return or his story well, and locked him in the basement of the village church to starve for over a year before his rescue by the party. He has been back in Strahd's service since the vampire's first battle with the party.

Dworic the Dwarf (deceased)

Dworic was a very grumpy-looking dwarf who the party frequently found spying on them back when Ethereal Plane spies were a regular concern. He was killed by Ayduin in the Tsolenka Pass.

Escher

Escher has the appearance of an exceptionally beautiful young man, and a somewhat ambiguous accent best described as "wants to imitate his master's put-on accent but hasn't quite nailed it yet." Though giving a somewhat foppish and irreverent air when the party's seen and interacted with him, he also slipped them a note claiming they had allies in the castle.

Eva (deceased)

Eva was a red-headed half-elf who assisted with the turning of van Richten, and who attempted to steal the Symbol of Ravenkind in Krezk. She was killed by Bodaway.

Feral Spawn #1

A young pale woman with dark hair who loved to bite and didn't talk much; one of the vampires at the Feast of St. Andral's. She was later seen terrorizing St. Andral's Church, before she seemed to come to herself and flee.

Feral Spawn #2

A young pale woman with dark hair who loved to bite and didn't talk much; one of the vampires at the Feast of St. Andral's. She was later seen terrorizing St. Andral's Church, where she was eventually killed by van Richten and the guards.

Foxilon (deceased)

Foxilon was a halfling seen assisting with the turning of van Richten. He was killed by Osrin in the Tsolenka Pass.

Imithren Orren (deceased)

Imithren is a half-elf rogue who Sylvain formed a parasocial relationship with. The party gained a great deal of insight into Imithren's past when they witnessed her chapter in the Tome of Strahd, in which Strahd found her alone, exhausted and dehydrated, following the murder of her lone companion Rose Underfoot by the local werewolves. Like Sylvain, Imithren was raised with a great deal of half-elf baggage; unlike Sylvain, this came from the parent who raised her. She was killed by Osrin at tuama beannaithe.

Krea Commonbrook

Krea was the youngest of the Reformers in Vallaki, a protege of Calorfin's. From the most recent group of adventurers — only a few decades before the party's arrival — Krea was turned along with Calorfin when Strahd "cleansed" the Reformation Centre. His current whereabouts and abilities are unknown.

Laucien Ariessus (deceased)

Laucien Ariessus was a badly scarred elven ranger who nearly destroyed Osrin's soul itself while she was awaiting resurrection after her death at the Fidatov Estate. His assault was only unsuccessful thanks to an intervention by Alandras. He was, funnily enough, killed by Osrin later on in the Tsolenka Pass.

Lizeth Reed

Lizeth Reed is a former resident of Waterdeep and former guard at Jewels of the Deep. She was abducted by Arrigal and turned as part of Strahd's strike against Sylvain. According to her, Arrigal made some effort to choose people who would be less likely to be missed, and targeted her specifically as a guard who never worked at the brothel at the same time as Sylvain, presumably in an attempt to lessen the blow of his orders.

Dr. Rudolph van Richten

In a cruel twist of irony and exceedingly on-the-nose fulfillment of a curse levelled on him by a Vistana woman long ago — there's a warning for you, Bodaway — van Richten was made a vampire spawn after his death during his attack on the Vallaki Vistani camp. Strahd, never one to pass up a tailored form of torture, assigned Strahd to protect that very camp in perpetuity, to ensure no other such atrocities can happen again.

Seward Morley

A former resident of Waterdeep, kidnapped by Arrigal and turned as part of Strahd's strike at Sylvain. Seward is married, with three children, and used to work in a warehouse in the Dock Ward. He told Sylvain he was turned because Strahd thought he would make a good fighter.

Sinariel (deceased)

Though human at first glance, the party later learned that Sinariel was an aasimar — the first to enter Barovia, and the only one aside from Osrin. Osrin herself killed Sinariel, though she was able to lay her guilt to rest after meeting with the former adventurer in the realm of the Huntress, where Sinariel thanked her for freeing them.

Toolcarver (deceased)

Toolcarver was a goliath paladin who the party first encountered during the Feast of St. Andral's, where he knocked out Bodaway before heading for the church, where he was killed by a disguised van Richten and various guards. The party saw him again in the Tome of Strahd, where they learned he'd been part of an adventuring party that fell prey to infighting thanks to corruption by Dark Powers.

Uthemar Hanali (deceased)

Uthemar Hanali Hanali was the husband of Helena Farlight, the former adventurer who guided Osrin in Mount Celestia when she sought Alanadras's counsel. While Helena was killed after the dissolution of their party — comprised of the couple, their daughter, and their best friend — Uthemar was made one of Strahd's servants, serving him for an unknown period of time before finding his opportunity for revenge by sharing information about Strahd with Savva Antonovich, the contemporary reincarnation of Tatyana Ostrovsky, a fellow vampire spawn, and Strahd's spouse at the time. The information he gave Savva directly led to their decision to destroy themself, a cause and effect that left Strahd furious. Uthemar's exact fate is unknown, but it seems he was no longer serving Strahd by the time the party arrived.

The Woman in Black

One of Strahd's so-called brides, the Woman in Black is a powerful caster who is never seen without her golden skull mask. The encounters the party has had with her are relatively few, but have left them not particularly eager to fight her again. Her voice sounds quite young, and quite mad.

The Woman in Green

Another of Strahd's so-called brides, the party has had the least experience with the Woman in Green, seeing her only during Strahd's claiming of van Richten's recently resurrected body, and in the attack on Jeny's cabin preceding the tidal wave. She has not displayed any arcane prowess. She is pale, with red hair — presumably an investigated and debunked Tatyana incarnation.

The Woman in White

Also known as Ludmilla, the Woman in White is calm, composed, and regal, with dark skin and excellent posture. She has displayed powerful arcane prowess, including the ability to render an entire room compliant with a Mass Suggestion spell. She was appointed to help rule Vallaki alongside the local priest, Father Petrovich.

True Vampires

To compare a true vampire to a vampire spawn is to compare Ri-An's AC to a zombie's. Though they are inextricably intertwined and share many common traits, a true vampire is a threat on a level incomprehensible to the average person. As van Richten advises, the best defence against a true vampire is to never go near one in the first place; for the average person, to be anywhere near a true vampire is to already be doomed, if it's set its sights on you.
Vampires are considerably hardier than their spawn, though not much harder to hit. They have exceptional Darkvision, and enhanced regeneration powers. Strahd, in particular, has displayed a wide array of damage resistances, including some seeming immunity to damage, though how much of that remains in the wake of the reconsecration of the Fanes is unknown.
van Richten describes how the powers of a true vampire are as varied as true vampires themselves, and to never assume that just because you've seen the powers of one vampire, you can guess the powers of the next. However, Strahd at least has demonstrated shapeshifting capabilities — a relatively common power of true vampires, according to van Richten, at least as far as his ability to become a bat or a cloud of mist go — and magical powers, though this is likely from the influence of Baba Lysaga and his own studies rather than any innate connection to his vampiric nature.
Strahd has displayed powers above and beyond what van Richten described in his book, though again, much of this may have been tied to his status as a Darklord or his desecration of the Fanes rather than his vampiric nature.
If a vampire drains the life of a humanoid with its bite and buries the corpse, the body will rise as a newly created vampire spawn bound to its will the following evening.

Known True Vampires

• Strahd von Zarovich
• Lord Percival Eggenberg, the unfortunately named vampire who set Azuth Crummond on his path of vengeance