How do you become a vampire in elder scrolls oblivion

To become a vampire you can do two things.
1st you can complete the dark brotherhood quests and then you have a choice of becoming a vampire.

2nd you can find a cave with vampires and let them hit you until you contract their disease.

i do not recommend beaming a vampire as people dislike you more and as you sleep, the effects get worse until its completely drains your entire health when out in sunlight. having said that, being a vampire does fortify all of your skills.