I thought vol'jin was just laying around for a little while before the player found him, if you get your throat slit you should be dead almost instantly and if you don't you won't be able to breathe and will die of lack of oxygen. Likely too quick for troll's regeneration to heal. During the razor hill questline it mentions that vol'jin's voice is still hard to understand because of his assassin run-in, so even though the player and chen did their best to heal him and even though the trolls have mad regen power, the cut was still severe enough to cause long lasting injury.
So once again, why didn't he die?