Vampires are people that are separated from the energy network, at least; that's one of the reasons one can be a vampire. Another reason is a leak in the aura, which makes the energy flow away. There are more reasons - many unkown - why one is a vampire, but I won't write them down here.
Since - whatever reason it is that makes one a vampire - vampires need energy, they can take energy from people around them. Actually, this happens automatically, when they focus their energy on the person that is to be drained, or when people focus on the vampire.
This isn't always easy, and so there is another way to feed: by blood.
It is full of energy, and they can drink it to restore energy (vampires are able to digest around 200% of the daily amount of iron, when in the form of blood).
When feeding, both energetically or physically, you have a special kind of connection with the donor. It is hard to explain what happens, but if the person is near you; you mostly sense it; if something bad happens, you sense it, etc.
I'm not sure how this connection is established, but I know it's there.
My personal theory, is that once you have fed on someone, you of course have some of their energy, and so actually a part of them. I think that's what connects vampires with donors.
Also, many people say there are psychic vampires and sanguinarians, however you basically feed with energy, and blood is just an energy bomb.
Vampires mostly live amongst humans, and what many fake vampires, or even real ones say, is we're vampires, not humans.
I don't believe this, because we are humans with some different this. Still human.
And calling a vampire?! They aren't some kind of entity you can summon!
If you want to know more, you only have to tell.