So far I have to say that I've been pretty lucky when it comes to the random vampire attacks, although I am only about four hours into the Dawnguard questline. I've had about five attacks so far, and they all have been right at the entrance of whatever city I am in which were easily stopped. Haven't noticed any NPC's dead because of this (which I probably just jinxed). If by chance the NPC did get hit by the vampire, I noticed they just drop to the ground with low health but do not die.