At level 57, I haven't had any
major problems with this as of yet.
I did get unlucky and get an Ancient Dragon plus the Vampire goon squad at once in Dragon Bridge, though. Had to reload. Was so snared from the Vamps' frost, and so absolutely-on-hot-damn-fire via the lovely stream of fire protruding from the dragon that every guard
and just about every civilian died.
Most of my vampire attacks have been on the roads or in the wilderness, though. Apart from the three asshats I got the very second I loaded up my save in Windhelm after installing the dlc.
All in all, not too bad, I suppose. It does lend a constant sense of urgency and paranoia.
However, I see a lot of reloads in my future. I simply cannot stand my npcs dying unwarranted. As others have said - Skyrim's population is far too low for this to go unchecked. Walking into a city or villiage and it being a ghost town is simply not cool.
I'd at least like for the attacks to dramatically decrease if you finish the story for the Dawnguard. However, seeing as this should have happened for Dragon attacks after completing the main quest.... I'm not holding my breath.