Bigmit37;
Did you go through all the speech options with the vampire before becoming aggressive?
Did you refuse to get George the Drudanae herb at the start of the game? That will break the murder quest, even if you later bring him his fix.
SirChronos;
You can also pickpocket (level 2 required, IIRC) a potion back from a cured person to use on the third, or kill one of them (no drop in reputation) so they drop the potion.
