Vampire butlers: As long as they don't have the weakness that prevents them sleeping in beds, sleeping in a bed will replenish their energy. I played a vampire butler for awhile and it was fine. You can't control butlers so there really isn't any way that they would have any powers or weaknesses.
Change their clothing: I think there is a mod for that now at MTS. It's called Hired Help.
Invisible trait: Butlers do have a hidden trait and hidden buff that identifies them as butlers. I don't know how you would add those to regular sims though, or if it would actually make them act like butlers.