I can attest to an increase in lung capacity after a year of vaping. I have some really insane mods, and can push rediculous amounts of thick vapor with no coughs. Let my friends try a few of my set ups, and they almost throw up. When I started vaping, I was about 3ml a day. Now I can go over 10ml. I do 2 miles a day, and that was the biggest tail tail sign that something positive was happening.
My lungs were fried though when I started. Almost 3 packs a day, constantly had a cig hanging from my mouth. 15 years of all the chemicals from the shop + smoking that heavily was destroying me. I had damn near permanent resperatory infections, and a smokers cough from hell. About 6 months in, it was almost completely gone. 9 month mark I had really started to master DIY mixing, and the cough completely went away. Missed flue season completely this year !
Maybe I'll die from some strange chemical in one of the 100 flavors I keep on hand (my only real concern is those chemicals). But at least I won't die coughing up blood, and no one around me suffers any ill effects.