Two responses here.
First: villain main character? No problem. Not by any means a requirement for me to enjoy a game, but unless you have at least a decent level of skill at characterization or some shortcut to elaborate to the player their PC's relationship with the game's ladies (incest themes, for example, come to mind), various levels of non-consensual sex are usually good ways to make the game interesting. So, sure. Villain away.
Second: Vampires? Much as your end-state sounds interesting (again, I fucking love incest), I've never seen a porn game do vampires well. I can by no means guarantee it's impossible, but I've seen multiple attempts and none of them yet have been any good, or at least the parts of them that didn't involve vampire stuff were the only decent parts of those games in my opinion. Regardless, though, if it's your passion then I think you still stand a better shot of making a good vampire game than one involving a premise about which you don't give a shit. So...bon chance?
First: villain main character? No problem. Not by any means a requirement for me to enjoy a game, but unless you have at least a decent level of skill at characterization or some shortcut to elaborate to the player their PC's relationship with the game's ladies (incest themes, for example, come to mind), various levels of non-consensual sex are usually good ways to make the game interesting. So, sure. Villain away.
Second: Vampires? Much as your end-state sounds interesting (again, I fucking love incest), I've never seen a porn game do vampires well. I can by no means guarantee it's impossible, but I've seen multiple attempts and none of them yet have been any good, or at least the parts of them that didn't involve vampire stuff were the only decent parts of those games in my opinion. Regardless, though, if it's your passion then I think you still stand a better shot of making a good vampire game than one involving a premise about which you don't give a shit. So...bon chance?