I'm not able to finish this quest, or do any other Vampire quests, because Vingalmo sent me to turn someone who was already a Vampire. The option to feed doesn't come up after Seducing them. Are there any fixes for this or am I out of luck?

this is the infamous vingalmo bug,so yes you are pretty much screwed,unless of course you want to kill the person,thus failing the quest,but what fun is turning someone into a vampire if your just gonna have to kill them?

Point... Well, it is just Winterhold, and no one actually cares what goes on there... I guess I'll wait for news of some sort of Patch before killing someone, since the next quest I'll probably have to kill the Begger in Markarth.