You mentioned that the amount of enemies, and their blood reserves, in the game are finite.
So I came up with another argument for having vampires using a coffin. Once a vampire is in the coffin his 'hunger-meter' stops. This way you can go to a town, put your vampire on a coffin and the rest of the party can go shopping, ponder over some crafting, read the journal and other town-stuff, without worrying that your vampire will soon be a ticking starve-bomb.
Instead of a coffin you could also give him some meditation skill.
