I get attacked by everyone, starting with the guy who wants you to walk up to the fort with him to join. Is this a bug? Or what?
EDIT: I am not %100 sure what happened but I figured it out and everythings good
You probably have a bounty in the Rift.
Even a Stage 4 Vampire won't get attacked even in the largest of cities. The only way being a Vampire gets you attacked is if you go into Vampire Lord form in the presence of other people/creatures.
I've found that since the Dawnguard are based in the Rift, and they're hopeless do-gooders, they won't accept you if you have a bounty on your head (hence the attacks).
Hope this helps for future reference.