Wait, there's a quest from the vampires to destroy the Dawnguard? I've only defeated her father and did a bit after that, that's all. Perhaps that's why...
It's a side quest apparently so you gotta talk to the random vamps in the court.
It has nothing to do with the stopping of attacks though I'm sure. I didn't do it and my attacks still stopped.
Vamp attacks definitely don't stop after you finish the DG main questline...it becomes part and parcel of the environment.
Dude it's 'SUPPOSED' to lol
People have been complaining to the "whiners" that the attacks will stop when you finish the MQ so they slap them on the wrist and tell them to focus only on the MQ and to shut up lol.
There have been plenty that say that completing the main quests stops the attacks.
HOWEVER, it can bug, and they won't stop no matter what.
Normally I say just because youre not bugged doesnt mean no one else is
But in this case, just because you're bugged doesn't mean everyone is lol