Vampire universe concept:
Vampires whose venom is what enthralls their victims. They're ambush predators that have to bite first to gain control over their prey's mind. The venom dulls fear and causes a sense of calm, so the victim is more likely to follow directions.
The venom also happens to be an excellent narcotic at a certain potency. So, there's a black market trade in a venom-based recreational drug harvested from captive vampires.
Authorities mostly look the other way. Who cares about bloodthirsty vampires anyway? They're being useful for once. However it becomes a problem if escaped vampires end up loose in populated areas. So it's technically illegal.
For extra spice, maybe someone discovers that the venom is more potent when a vampire is hungry or in distress. So they're kept just healthy enough to produce venom while still having a Very Bad Time.
