If you finish vampire quest and turn into vampire, can you go back?
Solution 1:
For a small fee (well, ~600 Gold), one Prelate Sabinus found at the back of the Mages Guild in the city of Evermore will cure you of your condition.
Solution 2:
For Aldmeri Dominion you want Prelate Sabinus in Rawl'kha, in Reaper's March at the back of the Mages Guild in the temple. She cures both Vampiries and Werewolves.