My MIL got lung cancer and she never smoked. She was a secretary for an industrial plant for most of her life, and got exposed to chemicals. (1 lung removed, she's a trouper, still going strong.) I've always considered 20 years as a fairly reasonable barometer.... That being said, I can't imagine that anything showing up (lung cancer and whatnot) would be reasonable to blame on vaping alone, if cigarette smoking happened prior, unless it was of very short duration. For example, my PCP wants me to go for another X-ray in six weeks, as I have some (what everyone assumes) is scarring in my left lobe, from smoking through a few too many pneumonias, but my PCP wants to be sure it isn't spreading into lung cancer, I'd be HIGHLY unlikely to blame it on vaping since it's been less than a month SO FAR, in fact even my doctor would certainly quantify it as smoking related, not vaping related, even to the CDC.
That being said, if vaping is less harmful than vaping, it's pretty easy to look at cohort groups over 20 years. *If* they find that vaping is harm reduction enough to substantially lessen the risk of smoking then I'd think that one would be looking for % of vapers getting negative outcomes after 20 years, versus smokers who'd smoked the same amount of time, to make the cohort even more precise, one could simply choose to compare former smokers still smoking vs vapers who STARTED smoking around the same time, then stopped. It's not a difficult study design, but cohort studies can only measure correlation due to the many other variables at play. They aren't dreadful by any means, they have to be well designed.
Anna