I have interacted quite a lot with people who identify as vampires, and have even written articles about them for people who know nothing about the subculture.
Essentially, these are people who are lacking something important in their body that they can only get through other people, and so they use any method they can in order to try and get this "something important" back. Some have found they can only get this from drinking other people's blood (which they attain with consent from willing donors), and some find ways to "feed" from energy. There are energy exchanges when anything contacts another thing, these "energy vampires" have just found a way to recognize these energy exchanges to boost their own energy levels.
Sadly, the real vampires are often seen as crazy by people who have never interacted by them. They know that their need doesn't have any evidence supporting it, but there aren't many scientists spending money of figuring out why some people feel the need to drink human blood.