This achievement requires the player to kill 1000 Vampire units in a single battle. Fortunately, Vampire units don't break and run away, so as long as the garrison/army is more than 1000 units strong, you'll get it. This can be obtained in both the Eye of the Vortex campaign and the Mortal Empires campaign.
You need to play as Court of Lybaras or your friend needs to be Court of Lybaras in a co-op campaign. For example. I was playing a co-op campaign as Khemri, and he was the Court of Lybaras. During his turn he attacked a big city with a full army in it. We played the battle together (by giving some of his units to me) and we killed all vampire units, some died from disintegration but it looks like those count too (i'm like 90% sure). After the battle the achievement unlocked for both players.