Well, speaking as an asthmatic who's been a successful vaper for over 3 yrs... the smoking really does need to go. If you mean "that other stuff," that too can be vaped --
http://www.seegovapor.com/vhit-series-vaporizer/seego-vhit-reload -- and it's a lot easier on the lungs that way.
I would also advise using higher PG than VG; if you've been using rescue inhalers for several decades as I have been, then PG won't even cause irritation -- not because rescue inhalers contain it, they don't, but your throat is used to inhaling something that causes irritation and dryness already; vaping PG probably won't cause anything worse. VG is very thick; it's like painting something with thick paint rather than thin paint. Thin paint accumulates, um, THINLY; thick paint accumulates thickly, and your airways can feel the difference.
VG is no more "natural and safe" than PG; both are created in a laboratory, and both are organic chemicals, which are actually somewhat similar -- they're both hygroscopic, for instance -- they'll both cause or worsen dehydration if you don't drink more liquids, you just won't notice it as much with VG, because of its emollient effect, so it could actually be worse for dehydration -- PG causes a palpable feeling of dryness, which leads to more fluid intake, which is exactly what you should do to counter that drying effect. (non-caffeinated and non-alcoholic)
Also, you should definitely stick to low wattage and high resistance, so as to create a minimal amount of vapor. Vaping IS a lot safer than smoking, thx to the total absence of microscopic "tar" particles and toxic chemicals, but asthmatic lungs will struggle if you keep trying to inhale insane amounts of vapor just to look cool.
And finally... commercial cigarettes contain chemicals which are there to be topical anesthetics -- so, you do worsen your asthma when you smoke, but you don't feel it as much -- BT wants people to smoke as much as possible, and the only way human lungs can do that for years and decades is by camouflaging the damage as much as possible. Giving up cigarettes means getting the topical anesthetics out of your lungs, so ANY smoke will be more irritating -- hence the link I gave above.
Vaping won't damage your lungs the way that smoking will, but it may irritate them, causing inflammation, which causes a "mucus" response, and without all those topical anesthetics, you're gonna feel that a lot more than you feel it, from smoking. It won't make your asthma actually worse... but it can definitely make it SEEM worse -- I had to start using Advair when I switched to vaping, though all I needed as a smoker was a rescue inhaler.
Andria