[quote=@Wraithblade6]
<Snipped quote by Witch Cat>

I was jus t thinking this myself. That vampires are magicalcreatures and need some magical aspect of blood, not a physical aspect of it, but needing cells or fluid is probably also useful.
[/quote]

Well, since vampires focus all their blood, and life force (don't have a better name for it) on keeping vital functions working, we could say they can't taste very well. Food becomes plain to them so vampires don't eat normal food as much (Atlas being an exception), they can lack several needed nutrients. So they feast on blood, the only thing left that has any flavour to them. To both stay alive, and stop their hunger, vampires feed on blood. Also gaining needed nutrients like iron to, y'know, not get malnourished. 

That's just my idea, we need to wait for GM approval of it.