I assume that the VM has been cleaned correctly on a regular basis and this cause can be excluded.but after using it as my daily driver for over a year, my chestmuscles/lungs are starting to be a bit irritated by the vapman the past few months.
(including filter screen and airways)
Could it be that the air intakes in the middle section are a little clogged?
They can be cleaned relatively easily with a pin.
Another possibility could be too finely grinded herb, which is also perhaps too dry?
Rarely I had strains with a, somehow, special terpene profile, which made me cough despite good temperature control.
(Badly grown grass full of fertilizer and insecticide toxins, or Brix, I deliberately exclude here, because we are all gourmets and pay attention to the highest possible quality, I suppose.)
Nevertheless, the vapor-filtering can be done easily through a small bong, which indeed makes the steam even more gentle, ....but also takes away a little of the aroma. (My guess is it filters out the very fine dust, which can create irritation.)
I use the vapor filtration especially on weekends, when we exaggerate at excessive steam parties and of course, we "must" absolutely try 10 new strains.
For the usual use of 1-3 pans per evening, however, there is no problem with my lungs. (and I am very sensitive)
I hope you find the solution that suits you.