Historical figures certainly had secret lives, but they are usually run-of-the-mill stuff like dubious sexual tastes, adultery, hidden illness, links to criminals and criminal acts, or a liking for controlled substances. I think it's stretching the imagination to think that any historical figure was a secret vampire slayer - though there may have been a few who fancied they were vampires!
I have to ask, does anyone really ever go to a movie with a totally open mind? How have we decided upon what we will see? If we were totally open to all movies then we would see everything going and even the turkeys would last for weeks and weeks at the local mutliplex. If we were totally open-minded then there'd be no use for reviewers and critics to help us make our minds what to see, and we'd have no personal taste. I saw the trailer for this movie (another thing to help us make up our minds) and it looked pretty silly to me, but then, like Destructor, I'm not an American and have no particular interest in Abraham Lincoln.
The again, if there were a movie called Winston Churchill: Vampire Slayer!!! - I probably wouldn't go and see that either.
But there's nothing wrong with going along and watching a movie and enjoying it if it's your cup of tea!