Sometimes the vampires dig a hole in the ground instead of burning to ashes. Still many times they just burn, go berzerk attack theri caravan membrs and become dust.

I am not about what causes this difference in behavior. I think one of the lord of the rims mods might disable the "digging" thing. It is a wild guess tho.

They come in day raids too, burn and die. Trade caravans too. Sometimes they can reach the colony before dying, Others they just die without digging a hole.

I bet it would be a pain to mod vampires to only show up during night but if you could make the digging work it would fix this issue.