If the DMT 'vapor' is harsh in any way, then too much heat is being applied or it's being burnt. It's a delicate process to get it to where it feels like nothing more than warm air entering your throat/nasal paths, there should be zero irritation from the vapor.
But on a sidenote - for actual smoke, like from enhanced leaf or changa, I always use near-freezing ice cold water, I might cough a tiny bit, but it's typically within a split second then subsides, near zero irritation from a thick cloud. Might not help everyone, though it's worked very well for me over the years.