On elder scrolls oblivion how do you become a vampire

get attacked by another vampire. talk to raminus polus at the arcane university to get the quest for the cure

sometimes you can sleep in vampire infected forts and caves if you have merrunes razor expansion pack go into merrunes razor near the end and sleep, and while you sleep you will be bitten.

There is also a quest that the dark brotherhood gives you and once you complete it you'll be given the option to become a vampire.