As a Vampire Lord, I was sent to two NPC's that I had already "converted". One in Markarth I was able to do again, but Valindor in Riften I could not. I used the Seduction on him while he went on about how good a favor I'd done for him. So, guess no more quests from the castle until I can figure something out.
Seriously running out of things to do even tho' I'm only level 52. Everything seems to be a repeat, even contracts for bandits from the stewards. And I've run out of "essential" NPC's to kill to blame on Dawnguard.
Hey! Bethesda! C'mon now, let's get on the stick and get something going here! :-)