Currently, as is, a squad of high level vampires randomly spawns when you travel to/enter cities and towns. These characters proceed to wreak havoc and slaughter the idiot merchants and misc quest givers of the town that rush them with iron daggers or less. Given enough (Not very much) time, this will leave the wide open world of Skyrim a lifeless ghost town, leaving many side quests broken.
The most simple solution to this problem is to make the attack a one time event, worst case is you lose one city worth of non respawning NPCs. Other fixes include making the spawns stop once the quest line is done (Can we get this for dragons too?), or making the townspeople smarter and guards stronger.