Here's my take on this.
Why has the vampire been asleep for so many centuries?
He had to go into hibernation because humans accidentally sealed the entrance to his resting place one day, so he couldn't get out any more. Until one fateful day when a lovely archeologist stumbled upon his coffin in a sealed off crypt.
Does my vampire sim need to be super ridiculously pale like Vlad? Or can he be a tad more healthy looking and not so gray?
At first he will be very pale, and will try to disguise this with make-up and get some moon tans and sun tans and lots of plasma of course. But after a while he will put his paleness down to being health conscious and too much work indoors.
Can my vampire become human again?
Silly question, of course not. But he might try to find a cure and who knows what he will find, and after being asleep for so long he's not quite tired of immortality yet anyway. And he's starting to get accustomed to and like the modern life and its trappings.
If yes, how would I incorporate that into my story so it makes sense?
Accident. He accidentally drinks some potion he shouldn't.