Well I can't seem to find Dworgyn anywhere, but I wanted to remind you that when you do Death School damage against Death school creatures, they will come a with a natural resistance to Death spells.
This could be why you're seeing less damage when you cast that spell. Try using your Prism spell before you cast a Death spell against Death School creatures.
If you want the prize, you have to build the ladder rung by rung. There are no shortcuts.
The most likely cause is that the enemy had a Tower Shield (50% reduction, it's a silver shield that rotates around the legs of the enemy).
Go to the Golem Tower in Golem Court. The Wooden Construct on the first floor has no shields or wards . Cast Vampire on it. The spell should do at least (and probably more than) 350 damage. If it only does 200 damage, then you need to send in a ticket to Support.
The spell has a base damage of 350. This will go up against enemies in certain schools (i.e. Life), and down against others (i.e. Death, as Zeke said). It will go up if you use equipment with attack enhancement (the fist icon; you can check this in your options), blades, traps, etc. It will go down if the enemy has a Tower Shield, Death shield, if the enemy casts Weakness on you, etc.
