Ok, so I am at the part in the end of the main dawnguard quest where you can do repeatable radiant quests for various vamps. I kept getting directed to turn people I already turned, so I reloaded the save and talked to everyone that I turned, then I went back to get the next quest. I was directed to the highelf vamp that always wants you to turn people, but this time the dialog never came up to where I can ask him what he needed help with.. it did with other vamps, but all they do is direct me to him.

Good news, I found out how to stop that one bug where you are asked to turn people you already changed.. bad news is now he wont talk to me about moving on to new people. I tried everything I can think of. My Argonian is lvl 60.. I refuse to make a new character.

I've got the same problem, where the option to start the radiant quest isn't there.
It will probably be patched sooner or later.