Wow. I'm really sorry that you had such a bad and frustrating experience.
*looks at watch*...wait for it....However!
I've finished the vampire side of the DLC in a slow 25-30 or so hours, and only a hand full of times did I notice vampires attacking a city I was visiting. When they did they only got as far as the front gate, and the guards were more than capable of handling them. Even against a Master Vampire. But in that case it was 2 v 1 + me...but I digress.
As a Vamp I know that I was attacked at nearly every cross roads by a pack of Dawnguard, and if I was near an external village that always multiplied by 1 dragon + 4 random hunters + 4 guards/soldiers + x villagers + the occasional couple of random wild animals. No joke. Serana saved my bacon more times than I can count. I'm now looking forward to see how the Dawnguard run through will turn out for me, but I'm currently leveling another toon from ground up for that.
I know none of what I just said helps, and if anything just frustrates your situation more. But for what it's worth, my perspective is that Beth's "random reality" always enriches my play through, even if it something happens that frustrates whatever current plans I may have. Those "WTF WAS THAT!?" moments are priceless to me. I would actually prefer to enter Whiterun one day and see a sea of vampires rauaging the city. That would be interesting and fun.
Think of the moral dilemma! As a Vampire Lord do I come to their aid or kick up my boots and watch the chaos?
=)
Cheer up!...could be worse...