I don't think you need a reason for the vampire to be asleep for so long. They are immortal. Time has no meaning to them. It truly makes no difference to an immortal being whether they sleep for 8 hours or 800 years.
I think it's fun, with occult characters, to think about what is normal for the occults-- the things they take for granted, that normal human sims do not.
It could be interesting to have a different non-human look for your vampire. Maybe yours is not gray, but another unusual color. Completely white? Completely black? Red or pink for a blood-related theme? Lavender or something? Or normal skin, but unusual hair and eye color?
(I like other people's suggestions on this question, too)