Lore is fun to dance with. If you wanted to skip the traditional Bram Stoker style, or (eww) Twilight sparkly vampires … and go dark yet untraditional with it …
Once NPCs are “fixed” you could, say, create a faction of crazy post-apocalyptic NPCs who believe salvation from the “plague” (as they see it) of zombie’ism is to drain and drink the blood of seemingly healthy humans. Maybe it helps, maybe it does absolutely nothing but they firmly believe its why they’re still around while everyone else is (un)dead. Wanna make them nutjob rogue scientists? … or maybe they’re a crazy religious cult? Maybe they augment themselves with CBMs and/or mutations that makes them seem semi-vampiric (sharp teeth/fangs, more claw-like nails)?
NPCs can be as crazy, wacked out and can even believe “wrong” things, lore-wise if you want (just good luck trying to reason with them). I mean, theres factions of people RL that believe all kinds of crazy, tin-foil hat stuff … imagine if the apocalypse happened and all the people that believe the government has secret moon bases where we meet with aliens to plan out the next Apple i-whatever release got together and formed a big biker gang.
lol … I actually am eagerly anticipating the NPC fix just to tinker with factions, gangs of NPCs and giving them crazy backstory and such.