Especially Vampires get some really nifty abilites you do not get othervise. Also, Durkon is as cleric a decent fighter and his spells... he can only use one per round and he is pretty high level, so he already has "most" of what he can get through spells.
Adding the template is very probably boosting him a lot, especially "in general", which means when he meets people who could not specifically prepare vs. a vampire. With Malack's "Protection from Sunlight" spell (something he has access to due to being a cleric himself) the biggest weakness of being a vampire is gone (and honestly, it makes no real difference if he gets a load of holy water as vampire or a bucket of alchemists fire as normal dwarf).