According to some vampire lore, it is not the blood itself the vampire needs to survive. It is the life force. When the vampire feeds, that is what they are draining. The blood is simply a symbol for that energy, and the act of drinking a symbolic gesture that transfers the life force into them. It is like taking Catholic communion. The wine is a symbol of the blood of Christ, and the cracker a symbol of his flesh. The ritual transforms these things into the real thing. Likewise with vampires, the ritual of drinking blood transforms it into the very life energy of their victim, which they devour. Because of that, a dead body is of no use to a vampire, because there is no life force remaining with it. It is like an empty six pack.
I have always liked this kind of vampiric feeding over the more trendy view of blood being like pepsi. Put it in the fridge for later and snack on it any time. That makes it too easy to be a vampire, IMHO. If someone is going to get uber supernatural powers, they ought to get some difficulties they have to surmount as well.