Many vampires are alive and living in the United States.
The more common kind are, of course, people in whom other people see vampire-like qualities. I used to have long black hair and fanglike front teeth, so in primary school another little troll used to tell people I was a vampire and try to get me to chase her and anyone who was currently claiming her as a friend. I never held a grudge...we all spent far too much wasted, minimally supervised, overcrowded time on a so-called playground that was basically an empty parking lot, and anything to do that wasn't actually fighting was an improvement. As an adult I've told friends that I eat garlic daily to keep my vampire side under control.
My husband's French-African-West-Indian ex-wife never had long hair or crooked teeth, but had more serious qualifications to be called a vampire by adults. One was that she never seemed to age; she had documentation of widely separated birth dates, and all seemed equally believable. More alarmingly, as a private nurse, she persuaded several people to name her as a primary heir to their estates shortly before they died--and the last two were young. Then there's the bizarre fact that although she was five inches taller, obviously belonged to a different "race," and had a heavy foreign accent, she managed to steal my identity...simple bribery, or shapeshifting? And although she was seen outside her home during the daytime, she always seemed to be chauffeured by someone else, shielded from actual sunlight...
But there are people who become so alienated from other humans that they seriously tell themselves and others that they're something other than human, at least "on the inside." Some of these people feel that they were born into human bodies by mistake, the way some people in the homosexual/transgender community feel that they were born into bodies of the wrong gender.
Live Journal hosts quite a few of these "Otherkin"; they have a community page that's open to public view:
My guess would be that several of these people were told a few times too many, as children, that "People" thought, felt, and behaved differently than they did, or that they weren't "acting like human beings." I'd also guess that the incidence of mental illness and brain injuries in this community is above average--the community page contains some posts from people who describe symptoms--but more of the group seem to be using metaphors to express a sense of emotional alienation. In addition to the vampires, demons, and space aliens, several community members identify their inner personalities as animals.
This is, of course, a different crowd from the LJ users who write speculative fiction from the aliens' perspective...we're ever so much more mainstream, although I sometimes wonder whether all writers haven't been made to feel like aliens during those bleak days when we were told that "Everybody" would rather be out on the playground, or pavement, trying to push each other flat, than be reading or writing, and we hadn't found each other yet and had no way to disprove this...
I can think of several good reasons for writing from the perspective of a rhinoceros, whether one seriously believes one's inner self to be rhino or human, but I'm less sympathetic to people who act out their vampire and werewolf sides by actually biting people. A relative of mine was so bemused by a girl vampire, when they were seventeen, that he let her bite his arms and slurp up his blood. Although he didn't realize it at the time, he was very lucky that he turned eighteen first and the vampire's parents threatened to prosecute him for statutory rape if the vampire didn't leave him alone. Fortunately it didn't take him very long to find a human to marry...I never heard what became of the vampire.