I reckon that there are Vampires in the world, thought they are not like they are in books or movies. I think that they are normal people with only a few of the myths being true. For example, I think that they are sensitive to Sunlight, and probably garlic, and silver too, but I certainly don't think that crosses or crusifixes would have any effect on them what-so-ever, unless of course, they were highly against the Christian religion, for some reason or another.
With the theory of them drinking blood, on the other hand, I'm not so sure about this one..
My opinion, is that they do, but only with the permission of their "victims", and I assume that they wouldn't take much, either. Seen as the human body can only drink so much blood, as it cannot be digested.

For a while, I believed myself to be one, as I can't look directly at the sun, I am allergic to garlic, though not silver, and I have an addiction to blood. This was why I used to self-harm for a while, not because I liked the pain, but because of the taste of the blood. I love it. Even now, I will often bite my tongue, just so I can taste it.