This is really easy to do. If you fight a battle against the Vampires, you will always get this, as Vampire units don't break/retreat. Build a strong army and destroy the vampires' army. Vampire units start to dissolve when other factions' units would retreat. This is no problem though and will only help speed up the process.
Don't forget to kill the leader!
After the battle is done all vampire units should be dead, and the achievement will unlock.