How do you become a vampire on elder scrolls of oblivion 4

To be a vampire I had to join the dark brotherhood (you do this by killing an innocent npc and a message will appear that's says something like "you're actions are observed by unseed forces" then do the quest) and getting so far. then talk to the vampire in the cheydinhal and he'll ask you if you want to be a vampire (idk how far you have to be in the guild just talk to him every once in a while) then he tells you to sleep on his stone slab for awhile. sleep there and when you wake up it'll tell you something about your character feeling sick or something.