[quote=@Witch Cat]
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Time to use our favourite tool to ignore problems, Magic! Here's my idea, all blood is infused with magic, Vampires need magic to survive, Vampires evolved to consume blood for magic, and that's why vampires drink blood. 
[/quote]

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.