How do you become a vampire in skyrim?

you need to be attacked by a vampire, the easiest way to get infected is to do the main quest and when they get you to go to azuras shrine she gives you a mission to kill 5 of her followers that were bitten while killing a vampire. Each time that they attack you theres a small chance that you'll contract the disease polymorphic hemophelia, once you have that disease dont treat it for 3 days and go to sleep, you have nightmares and wake up a vampire

But what if you have been bitten by a vampire in the darkbrotherhood (Vicente Valtieri or as i call him vinny) and you have waited 3 days and then slept and nothing happend what should u do? its happening to me and im getting furstrated >:(