Necromancer – “Invasion” [Campaign 3, Mission 3]

Vampirism can also make the effect of raising permanent.

Absolutely correct. If you apply "vampirism" to a stack of living creatures and then raise the dead, the raised portion will not disappear. This combination of spells is equivalent to "resurrection." If the living stack is completely eliminated and a portion of it is raised as undead, then with "first aid" at the end, a small amount will remain, regenerated by the tent.