In Oblivion, being a vampire only gave you some stat boosts and an altered skin tone. there were a million ways one could have boosted their stats without having to burn in the sun for it.
So in Skyrim, if vampires are in, then there should be some unique advantages to being a vampire or werewolf. whether it is a game mechanic or some sort of aesthetic flare that you can only obtain by becoming a vampire and suffering all of the burdens that came along with it. some tangible ability or appearance that could not be mistaken for anything else.
So, what could some of these advantages be?
an easy one to implement would be a few unique kill animations where your vampire character feeds on his enemies.
or perhaps increased speed since athletics is no longer in the game. nothing game breaking of course.