I often use the firestorm scroll. That seems pretty effective and I bet it would work great on vampires. The main issue, I suppose is that it takes time to activate.
True, although if you throw an atronach between you and the vampires, you can be charging it up as your summon takes some punishment. If you don't know the spell or don't use conjuration, there's the possibility of either looting or buying a Staff of Flame Atronach.