[Renojustin]

I can think of few counters to this but hate it or love it, one is quite broad - any nation with significant access to Fire magic and Fire-resistant or immune troops will smash into any unsupported vampire colony like a hot knife through butter, provided that it is not yet into the late game.

I also like to flavor my fire with death so it would probably have the effect of blunting the edge of your knife as even the immortal, regenerating vampires accrue heavy afflictions after one or two fights against a fire mage with a shroud and death bless casting BEs.