Hypothetically speaking, if I were a chemist, it wouldn't matter. Perhaps I am cynical, but I believe that very often research reflects the biases of whoever will fund it and furthermore independent research is often seen as non-credible just by virtue of the fact it is independent, especially if it fails to confirm dominant notions.
Basically, science has become politicized and is no longer objective.
So pretend I run the exhaled vapor through a mass spectrometer to determine its constituents (qualitative); and then use high performance liquid chromatography to determine how much of each (quantitative) ... what would it mean? Those results would apply to just that particular e-cigarette using that particular e-
juice having run through my particular lungs. So anything I put forth would not be generalizable.
So pretend I did something similar with 1,000 users each of every conceivable e-juice and e-cigarette combination. (There could be a difference in constituents based upon changes made by the varying atomizer temperatures, etc.) After that, I would have a pretty good idea of what was in the exhaled vapor.
But what if my results said that the exhaled vapor were even more dangerous than 2nd hand tobacco smoke? (Remember, we're just playing pretend.) What if they showed that the generally-recognized-as-safe (GAS) ingredients in e-juice, when subjected to a hot atomizers become highly carcinogenic substances etc?
I know what would happen: the e-cig industry would shout me down and I'd be called a crackpot pseudo-scientist etc.
And the exact same thing would happen if I showed it to be rich in beneficial cancer-preventing anti-oxidants. Detractors would go nuts.
Basically if, hypothetically, I were a scientist, I would consider such an endeavor to be a waste of my time unless I were getting a government grant to confirm an existing politically correct bias; at which point my research would yield the expected results anyway.