Villain Code

If SIVIC is reading this, it doesn't exist and you're all buffoons for trying to crack the code and you should be eternally embarrassed. Same goes for any prominent superheroes.
For everyone else, of course there's a secret language that supervillains use to communicate. No, you can't take a class on it. And no, figuring it out doesn't grant you access to some secret aspect of society. It's just a way to talk to each other and leave notes without having to use more official channels that could be observed.

Entirely Unspoken (Or Is It?)

VC is primarily a language of symbols. Not exactly written, but that is probably the closest approximation. It's designed to communicate nonverbally, occasionally through hand signals but primarily through the use of positioning within buildings and paintings and other public symbols that can then be translated by those in the know. Any verbal communication comes in the form of references (and they better be oblique references) to those symbols.
This is because VC is more about warnings and alerts than any sort of amicable conversation. It's not usually meant to be used in person, but left behind so that someone else can see what you were doing and react accordingly.
Most of the symbols have layers of meaning to them, historical context that informs at least part of their creation process. The majority of people don't even need to know most of that history, or the little tweaks that are made simply because someone thought it would look better.

A Few Examples

For the sake of example, I'll use one of the older symbols that has since been replaced.
The symbol meaning "I defeated X here, and Y is how" consisted of three flexible parts. X part was deemed the least important since the specific hero is less important than the rest, so if you can't fit it in or don't have time for something, this is the part to lose. The important part is knowing the precaution to take, not necessarily the specific person who you need to take it against. And since we all know the shorthands for superheroes, I'm not going to share those.
The primary symbol communicates, in its most concise description, "defeat of other" and "method". In this instance, the symbol required copious amounts of green and gray paint. The marbling pattern is difficult with most kinds of paint (hence why the symbol fell out of use), but it blends nicely with the concrete wall required for a base. The wall can be located anywhere, but it must be free-standing. Most people put in a separate wall next to an existing one. The pattern of the paint can be anything.
To convey the method requires a sequence of precise color tones painted in circles across the center of the wall. The positioning and nature of the circles doesn't have to be exact, but the colors must match exactly within parameters. If additional details are needed, most people put those details on the opposite wall, which requires no preparation.
The primary signifier that this is a VC symbol and not very random graffiti is the positioning of a light that doesn't turn on but points at the wall.