I defeat the Vampire in the Dark Cave when I was at least level 20.
It's strange that the Frost spell did not work, but IF you attack with a weapon that have Frost on it, it works !

Try with Poison damage or take the skill Poison Weapon, great skill!

... sercainlt if yo want to kill Orcs (mostly with a bow and Splittign Arrows very great !)
My
Bastard Sword of the Brain have many kind of extra damages,
like Frost(2), Poison Damage(2), Life Drain (3) and Mana Drain(3).