I've had one vampire attack since I downloaded the DLC, and it was as soon as I started the game with it installed.
Fast travelling definitetly seems to cause it, as I never fast travel and haven't seen any since.
Maybe fast travelling to the stables instead of into the cities would prevent this?
I generally fast travel to dungeons close to the cities before I enter them now just to be safe. As far as I can tell, vampires only spawn randomly in the cities if you fast travel DIRECTLY into the city. So far no vampire attacks since becoming a Vampire Lord on my Vampire, and about six on my Dawnguard character.
http://www.youtube.com/watch?v=v80gUzq6Ec8&feature=BFa&list=ULVEWKVNQwou8
It's a video about a vampire attack on Riverwood, with the death of Gerdur only. I have yet to do a full search for any other videos showing a more wide scale slaughter, but it does highlight the problems with the NPC AI dealing with threats.