Vampires. Discussion.

What do peasants have to do with it? Skeletons, archers, and 5 liches – and victory is yours.

You'll have 25 vampires and 5 liches after 3 weeks (or, more accurately, 32 vampires and 10 liches, plus 6 Undead). But even then, not all the neutrals will be killed. Unless the hero actively casts spells like Dispel, Blind, Slow, and Phantom. But the vampires will hardly be involved.