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 ! smile Try with Poison damage or take the skill Poison Weapon, great skill! biggrin ... 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).

On 7th of february 2015 : I start a new adventure in the Divinity world of Original Sin,
it's a Fantastic Freaking Fabulous Funny ... it's my All Time Favorite One !