It's just a thought - not yet evolved enough to really be a theory. Part of it stems from my belief that the human race is constantly evolving, and things like the "queer gene" or vampirism are probably normal mutations that stemmed from some outside stimulus - okay, the ground might be a little shaky for the homosexuality argument still, but I am of the opinion that bisexuality is probably as behavioral as it is orientational, but there are so few people that absolutely prefer the company of their own kind that there should be some sort of genetic stimulus. Vampirism is a little easier to explain - how many vampires see themselves as Otherkin or as not quite human at all, because they feel so different from everybody else? The line between human and humanoid is apparently slightly fuzzy.
Another reason I suspect genetics might be involved is that vampires seem to be susceptible to certain physical weaknesses (at least, the ones that I've polled) that might be genetically linked. Sun poisoning or photosensitivity, for instance - my mother had it, I developed it in my early twenties. I'm getting more and more of her allergies. She's slightly manic-depressive, and so am I. She has rheumatoid arthritis, and I developed that a few years ago; I have a much milder form of it than my mother, but it's there. I have terrible sinus problems, that sometimes trigger migraines; so does my mother. My mother's side of the family tends to be very long-lived, especially the women. They tend to live into their nineties and age gracefully.
I have some health problems that my mother does not: mild chronic fatigue syndrome (triggered by recurring bouts of Epstein-Barr virus), slight allergic athsma, food sensitivities, severe migraines (much worse than anything she had), amenorrhea (if I don't take the Pill, I don't bleed, at least, not very regularly. Sometimes I skip one month, sometimes I skip eleven months), psychotic PMS - my hormones are pretty badly messed up. Most of my health problems can be attributed to a hyperactive autoimmune system: it's almost as though I am allergic to myself or something. "Various connective tissue disorders of one sort or another" can also describe my problems. I was actually terrified this summer that I might have systemic lupus, because I showed nearly all the signs of it.
Anyway, back to vampire disorders. Connective tissue problems (arthritis, Sjogren's syndrome, lupus, CFS, fibromyalgia, etc) seem to be a fairly common complaint, as are photosensitivity (which might be a secondary symptom of some of these disorders) and autoimmune problems (immune system either works too well, or not well enough, or both). Migraines also seem common. All these ailments have a hereditary link; the parent might not suffer from the disease, but somebody in the family did and there's a gene passed along at the very least.
The fact that so many vampires (and Otherkin in general) seem to experience these problems indicates, to me, that vampirism might be genetic. I think we're mutants. There's a recessive gene that causes vampirism, and if Otherkin tend to inbreed because we understand each other better and are more attracted to each other than to "average" people, then of course the recessive gene is going to become more likely to surface - more likely to become a dominant trait, eventually, given enough time. It's a slow process. then again, think how long it took for Homo sapiens sapiens to become a species. The inbreeding and so on has to start somewhere.
Another possibility for the outbreaks of CFS and other connective tissue disorders might be that those of us who are sensitive to the environment are reacting badly to negative stimuli - we're basically allergic to modern life. When I visited a remote village in Scotland, where there was no pollution and no noise and time seemed to stand still, I felt healthier than I had in years. So maybe my hypothesis about the vampire gene, based as it is on shared health problems that might be hereditary, is all wet.