If you went through all the speech options with the vampire, you should be able to report back to Mardaneus to complete the quest.
Some people have had trouble doing this if Mardaneus was in the council hall, and refusing to get George the Drudanae herb at the start of the game breaks the murder quest, even if you later bring him some. If that is the case, there is a fix;
FIX for broken Imp Council Member quest 
