I figure it's more than just needing iron for vampires. It's like they have an extreme dietary restriction where they can only eat 1 food. The real question is why is that so bad for a modern vampire setting. Just buy blood from the butchers shop?

>>85851490
it depends on the fantasy, but in a lot of them vampires find the taste of animals disgusting or it degrades their magical power to do so.

>>85851503
Ok let's assume it's only human blood and it's modern. What would happen if a vampire was honest that they could only drink blood? Like they went and got studied and the doctors concluded too that yes this person can only drink blood. Wouldn't they just... sell you blood?

>>85851529
well, it depends on the setting again
in vampire the masquerade they have a secret society that keeps the existence of vampires a secret by killing everybody who finds out and any vampires that jeopardise it.

but seriously, do you think the existence of cannibalistic humans would be permitted in society? it would be like trannies but 1000x more controversial. Plus, most of the time vampires are overtly demonic entities that are inherently aligned towards evil. it wouldn't make much sense for humans to want to cooperate with them. a lot of the ideas you're having as sort of paralleled in tokyo ghoul though, worth a read if you havent.

pretty sure the lore is that the blood has to be fresh from the source or else it's harmful to drink