Get another vampire to bite you then you will get the disease wait a few days and you'll be a vampire. Being a vampire sucks though and is hard to cure so i'd suggest you create a back up save file first.
Well if you wanted to get bitten go to the vampire who when you get far enough in the quests for the dark brotherhood you will be offerd the 'dark gifft' just accept and the next time you sleep you will be bitten.
But also if you fight vampires you can purposley get striked a couple of times so you contract the diesease go to sleep, wait a few days and sleep again, but as zackman said its not worth it, theres no real benefit in it.
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