My standard work-around has been to save at the entrance of Castle Volkihar before entering when finishing a quest, getting the new quest and then ignoring it - it has been my experience that the game flags your next quest when you enter the castle. Today it kept sending me to Vingalmo and he kept giving me the quest to turn an already-turned vampire. As has been discussed on another thread I killed the turned-vamp, failed the quest, and returned. A new problem arose that I was told to speak to Vingalmo (again) and it did not register in my Misc. Objectives, poetentially breaking the Side Quests. Here's what I did:
I re-loaded at the entrance and again received V's broken quest. I failed it, returned and (after several tries) was told to visit another quest-giver, though it did not register in my Misc. Objectives. I went to the quest-giver anyway and was able to get a new quest.
The issue is you may have to keep reloading several times. Not sure if this will fix every problem with Vingalmo - the game seems to decide at some point you have to take his quest and won't offer any others - but it did work this time.
I hope this helps those of you who keep bumping in Volkihar Side Quest problems.
Edit: spelling etc.