There seems to be a vampire masquerade, but it turns out vampires don't exist... werewolves, however, do exist, and theirs is the true masquerade.
Parodied: Vampires look absolutely nothing like humans, but continue pretending to be human in normal society while enthusiastically stating at every opportunity that they are absolutely not vampires at all.
Zig Zagged: No human knows that the vampires exist... until we meet Alice. Then, however, it turns out that Alice is completely delusional, and although she thinks there are vampires, her beliefs are completely unfounded. Until we find out about the second group of vampires that exactly matches Alice's descriptions, of course. Then, however, it turns out that these vampires are actually just humans pretending to have supernatural powers. It turns out that this was a smokescreen to conceal the masquerade the werewolves are upholding.
Vampires exist, and the humans know it.
There are no vampires.
Enforced: The show was originally plotted to be about a human vs. vampire conflict, but the script was rather bloody, and after some Executive Meddling the writers had little choice but to remove the conflict entirely by having the humans not know about the vampires.
Lampshaded: "Can't we just tell the people that we're vampires? I'm sure not everyone would freak out about it."
Invoked: As Bob and his vampire family move into a new town, Bob insists that this time, they aren't letting anyone know about their supernatural status, because past experiences have been largely negative.
Exploited: Alice, a human, realizes that if there were vampires, they probably wouldn't let humans know about it, so she makes sure to vampire-proof her house just in case.
Defied: Bob actively breaks the secrecy of the masquerade and reveals himself as a vampire to the humans.
Discussed: "Oh crap, I'm a vampire. Does that mean I'm supposed to hide it from my family or what?"
Conversed: "I swear, half of the episodes' plots could be resolved if the vampires would just man up and reveal themselves to the townspeople!"
The vampires' refusal to show themselves to humans is a sign of their elitism and snobbery - they see themselves as better than typical humans. This leads them to refuse to ask for human help when crisis strikes them all, and they suffer greatly because they will not let themselves integrate into the thriving normal society.
A vampire breaks the masquerade, and the resulting consequences are significantly worse than the sacrifices made to uphold it.
The masquerade is actually bad for humanity since it prevents them from knowing how to protect themselves from supernatural threats and prevents any checks on the power of whoever is keeping the supernatural a secret. Those who are in the know have an effective monopoly over all interaction between the human and supernatural world, which not only lacks any form of transparency but also restricts any scientific or economic benefits humans may gain.
The development of information technology, such as the Internet and surveillance, breaks the masquerade since there is no way to keep anything a secret anymore.
Reconstructed: The masquerade exists for the benefit of the supernatural world, who are terrified of humans hunting them down. They'd rather live inside a prison of secrecy than risk what might happen if their existence was made public.