Honestly, I don't think she even thinks this much about it. She just runs with the Rule of Cool and to hell with it making any kind of sense in the narrative, let alone how it fits into the established worldbuilding. She's A-OK with retconning whatever she feels like which is why everyone's age and height fluctuate wildly and we suddenly have zombies who are murder victims going on a vengeance murder spree when previously they'd been raised to testify in court without any issues.

I totally agree that it'd make more sense if vampires were only recently out of the coffin! Or even something like Kim Harrison's The Hollows where there was a tomatopocalypse and turns out vampires are immune. It definitely fits with how LKH talks about "what if you woke up tomorrow and monsters are real? How would you deal with that?" Because yeah, the more I think about having vampires, zombies, and werewolves always being known, the more I think about how things should be incredibly different. But at least it's a good exercise for my own worldbuilding so I can avoid falling into the same traps.