My best friend's friend is both an energy vampire and blood vampire. What people don't seem to understand is that common legend isn't always true, and shouldn't be taken for face value. Vampires are -not- descended from bats, and you do -not- become one if your bitten..... and there are actually almost no cases of a vampire biting another person!
Vampires feed on blood for it's energy, since they themselves have an energy defitiancy. This certain type of energy can only be found in normal energy fields and in blood. For this reason, there is actually a "doner" system for vampires to pair up with people who want to let them have their blood (don't expect to find one happening near you... check out the internet though). This, really, is out of compassion, and the relationship is a good one!
A vampire will become very sick (and there are a few cases of death) if they have not had the energy they need for a few months. Yet, they only need a small amount of this energy on a monthly bases, and it's certainly not enough to kill the doner! The amount varies from vampire to vampire, but overall quantity is the same.
For those wondering, I -am not- a vampire, I am simply one who understands and knows one. I've done some extensive research, talked to actual vampires, and discovered the truth behind it. It's actually extremely fascinating!