If vampire attacks can't kill merchants, etc, then they would be even lamer then dragon attacks, which it shouldn't be. You might as well say that dragons shouldn't be able to kill important NPC's.
If you take that away, it would just make vampire attacks lame because there are really no severe consequneces in failing to stop an attack, since the important NPC's wouldn't be able to die.
I'm not saying NPC's should rush at the vampires and die, i don't care if they run away, however making it so that they can't die at all is stupid.
Here is my idea, non-guard NPC's should run away if they have iron daggers, however if you give a strong weapon to a non-guard NPC, they won't run away but stand and fight. That way, even if all the guards are dead in a villege, there can still be some defense against the vamp attack instead of them all just relying on you to defend them.