Blood Tenebrous
Vampires have little to do with spirits and the Shadow Realm, as a general rule. The vampires’ world is more strongly rooted in the physical than mages or werewolves — after all, that’s where the blood is. The Discipline of Blood Tenebrous provides an avenue for vampires with the interest to interact with the spirit world. At the troupe’s discretion, Blood Tenebrous may be a unique Discipline tied to a hitherto-unknown bloodline, one that walks the shadows and treats with spirits. This bloodline’s parent clan is most likely Mekhet (who else but the Shadows?).
There are other options. If you want a chronicle that ties vampires’ very origins to the spirit world, Blood Tenebrous can be made a common Discipline, not in-clan for anyone but available to all. In general, the source of this Discipline can be placed with almost any faction you choose; spirits are in all places of the world, leaving vampires’ interaction with the a matter of circumstance.
There are other options. If you want a chronicle that ties vampires’ very origins to the spirit world, Blood Tenebrous can be made a common Discipline, not in-clan for anyone but available to all. In general, the source of this Discipline can be placed with almost any faction you choose; spirits are in all places of the world, leaving vampires’ interaction with the a matter of circumstance.