Before becoming a vampire, it is recomended that you become a member 
of the Mages Guild, have access to the Arcane University, and either a 
member of the Dark Brotherhood or started the Order of Virtuous Blood quest.

Dark Brotherhood Story: Become a member of the Dark Brotherhood by 
killing an innocent person, going to sleep in an inn, traveling to the 
Inn of Ill Omen, and killing Rufio. Travel to the Sanctuary and talk to 
Vicente and finish all of his contracts. He will then ask if you wish to 
become a vampire. Say "Yes", then sleep on the slab. When you wake up, 
sleep again for twenty four hours three times. You are now a full-fledged 
vampire.