FWIW, I had a very similar problem in Riften. No bounty, not a vampire, but the guards still went absolutely pants-on-head bonkers for no apparent reason and tried to kill me. ...So did Louis Letrush. wtf, man. http://www.gamesas.com/topic/1331838-riften-goes-bonkers-and-spawns-hostile-stormcloaks/page__fromsearch__1
Painkiller figured out a fix for me: go to the nearest non-hostile guard and blatantly commit a crime, like pickpocketing them. Go to jail and serve out your sentence, then return to the problem city and see if the guards are still hostile. Doing that fixed Riften for me...it's like the game decided I had some sort of uber-bounty in that hold. Except that I didn't, and had done nothing that would have resulted in a bounty like that, either. I still don't know what that was about.
I'm not sure how well this will work for a place like Riverwood, though. You'd think if everyone there was hostile because of a "surprise invisible bounty," everyone in Whiterun would be too, since they're in the same hold. But you can try.

Hope it works.