So, if I side with the Vampires, my towns don't get attacked? Or do they just not attack me?
Vampires travel in packs in the wilderness at night but they don't attack you.
Only scripted enemy Vampires attack you.
You know the ones the Volkihar call Thin-blood vampires found in caves.
Dawnguard attack you in the cities though its rare after the main quest though..the towns people aid you over them which is funny.