Have you ever heard about the power that your mind can have on you. This is a phenomenon that is used sometimes by doctors. Here is a story I heard; A patient had a cancer and they gave him pills which had nothing medical and told the patient it would cure him. It worked, the cancer ''disappeared'' and the patient was healthy again even if the pill had no medical uses. It happened all in the patient mind. He believed so strongly that he was cured that it actually cured him! So, people may believe they are vampire and then have the same symptoms of vampirism (Fear of sun, garlic repulsion, blood craving, stronger sexual craving). So nothing magic about that!
Now this is the reason for psychic vampires. If the host thinks it is true that the one that claims to be vampire is one, he will feel tired and exosted. But that doesn't mean anything. The other explanation is that the person is really depresing and that does affect poeple's mood and reaction. Only normal. Anyone depressed as this affect.
I'm sure that lots of people that claims to be vampires is related with my explanation above. But what of the rest? Blood exchange, cutting themselves with knife and then licking or sucking their blood. These are form of Sadomasochist. Why not, what is different between getting whipped and cutting yourself. This might not pleased the ones that claims to be vampires but still it just might be true.