I'm not too familiar with this game so forgive me for being a bit blonde here but I will do my best to answer
From what I have read, in order to no longer be a vampire you need to
- locate a priest of Arkay. These are typically found within the Mage's Hall of the same city you received the request. The priest will gladly cure you, for a small eso gold fee. Please bear in mind that the abilities used are locked in place , so if you happen to turn into a vampire in the future, those skills will return and be ready to use. In order to retrieve the skill points used for vampiric abilities, you need to return to the shrine (found in capitals of each faction) for a respec.
There's also a video which might help you further, so I have included it for you. I really hope I have helped.