Oboy, another scientific theory debunking the existence of a monster, which doesn't seem to grasp that the "movie version" of the monster -- which the study is usually about -- has precious little to do with the version of the creature people actually believed in. (This is a crucial difference; "folkloric" monsters exist as conjured up by people who were honestly trying to explain the world around them, and thus tend to have fairly in-depth rationales when you go digging, whereas "movie" monsters are conjured up by people who are looking to meet the needs of a plot.)
As with the perennial "wraiths should destroy the world" threads, it also simply ignores the presence of opposing forces in the creature's ecosystem. I'm sure it would be no more difficult to mathematically prove that all life on Earth will soon be devoured by an ever-deepening sea of ravenous beetles -- simply by ignoring all checks on their population growth, such as its position on the food chain.
(Other posters have already hammered home that only a small fraction of people killed by D&D vampires would rise again as vampires themselves.)