[Radek]

I do not have Oblivion but in Morrowind, it was a matter of a few global variables and one journal entry.

Therefore, start the game editor, read scripts realated to vampires until you find out what happens when you get diseased and become a "candidate" of vampirism, and do the same using a console.
Also, you will need to remove your immunity to vampirism most likely. This should be another script, where you should find variables and journal entries, which you need to unset.