Most fairy tale monsters do have a shred of truth behind them. One of vampirisms can be linked to a extremely rare blood disease in which (I believe) the white cell count decreases a hugh amount and for some reason the drinking of blood replenishes it to some degree. Unfortunitly, I can't remember what the disorder is called, but I do vaguely recall a claim that Rasputin (the mad monk) may have had this problem.
Because this was seen as so monsterous the idea of evilness could have easily been attatched to it and developed into a relationship with creatures
that are seen as evil (aka bats and wolves). Just food for thought.