if you are a member of the dawnguard there should be a dawnguard stationed at each town/city
you speak to that dawnguard member to recieve a random vampire attack misc quest which would occur in a random town, once finishing the mission you get a reward
player: got any news?
dawnguard: i have been told that solitude (or any other town) has been assaulted by vampires and they need backup
the vampire lords will be given a vampire attack misc quest by another vampire lord who fears that the vampires are killing off their feeding grounds. this give the vampire lords a good enough reason to protect the towns
player: got any news?
vampire lord: vampires have been attacking windhelm (or any other town) we cant allow them to kill off our feeding grounds, deal with them and i will reward you
any other suggestions will be appreciated, thanks