If blood is needed to exist, it becomes essential medicine. Preserving life is more important than the kosher laws, so it's not a problem at all. (Vampires would have an obligation to take a small amount from each donor, so as not to risk killing the donor...preserving life is not more important than the commandment against murder. And the ethics of consent could get very interesting.) Laura Antoniou does some interesting things with the idea of blood and kashrut in "Shayna Maidel."