Critical Damage is calculated, and applied, first in the damage equation.
Example. A goblin grunt rolls 3 dice and gets a blank, 1 critical and 2 normal damage.
I take the 1 critical and apply it to the creature. Then I subtract 2 damage from armor (if any) and add apply that number (if positive) to the creature.
At any given point and time, if the damage on a creature equals the life total, it is destroyed. So it is possible to kill a creature / conjuration with critical damage before normal damage is applied. Not terribly important right now, but you never know....
As for your question, the veterans belt does not care the source of the attack only if the damage is critical or not. When you have vet belt equipped, simply take 2 damage from the critical and apply it to the second half of the damage equation as jacksmack said.