I look at it via the nail polish/paint metaphor. If you use a thin nail polish, it takes a lot of coats to build up to the point that it's opaque. If you use a thick polish, then one coat is usually enough, and it stays wet/tacky for a long time before hardening, and it's very thick; you can apply a second coat only long after the first one has dried, because applying fresh wet polish will soften the underlying thick coat, and you will end up with a VERY thick coat of polish.
People with asthma already have compromised airways, and what happens in an asthma attack is that the smooth muscles (the involuntary ones) go into spasm in the airways, constricting the airways, and the only thing that causes them to open again is to inhale the topical bronchodilator, which works on smooth muscles to relax them (I had to learn all that about my medication's effect on smooth muscles when I was pregnant and preparing for labor/delivery -- and as it happened, my smooth muscles did not work correctly during that event, and I required a caesarian).
When you inhale something thin, then it takes a very long time to build up to a point where it actually narrows the airways, and over that time, your lungs' natural action is to clear foreign material, so it never builds up to a point that it causes problems. However when you inhale thick material, it narrows the airways very quickly, more quickly than compromised lungs (compromised either by asthma or smoking or both) can clear it away, so that if an asthma attack strikes (normal for sleeping actually, due to shallower breathing), there is then very little room in those airways for air, or even to get medication into them. I can actually feel this build-up, thanks to that hyper-awareness you mentioned, and it feels AWFUL, like there's something that desperately needs coughing up, but will not move.
Thinning VG with water *will* work to prevent this thick, airway-clogging buildup, however it negatively affects taste and vapor production a great deal more than thinning with PG.
Andria