Well, I'm not a chemist, but I am an aerospace engineer (rocket scientist) and familiar in many ways with the process of what's happening here, as I think fluid mechanics is more important to what's going on here.
I would suggest that to say nothing is burning, just vaporizing, is an incomplete statement-- in fact, I think that when an atty heats up and you get that burned taste, either because the atty is too dry or too hot-- then there is burning there, and that's where the burned taste comes from. When the atty is operating optimally, you're getting pure vapor, but attys are cranky things that perform different depending on the juice you use.
I'm less concerned about contamination from transport-- I think a lot of people underestimate just how much of their silverware, dinner ware, cookware, clothes, etc. come from China. The raw bulk of shipments would raise the number of such incidents, but for the most part people don't think twice about buying a toothbrush that was actually manufactured in China, or set of pots and pans, etc. We're in no more danger there than we are when we buy a set of flatware made by a Chinese company.
I am more concerned about the flavorings used, but not for the toxic effects of natural flavorings. I think the physical chemistry of what's going on in the atty is pretty dependent on the physical chemistry and fluid mechanics involved, but to keep a long story short I think that the flavoring is what causes atomizers to gunk up and eventually need cleaning and/or become unusable. I think that when the juice does gunk up/burn, it's the sugars in the natural flavoring caramelizing and burning, or the artificial flavorings burning/caramelizing doing the same (fake sugar melts and burns too, the melty part just doesn't taste good like caramel).
That said, I think the carbon monoxide production from that process is likely to be extremely low. Any time you burn a carbon-based flamable, some CO is produced-- from burning wood, to burning your dinner in your kitchen, to lighting a match or a lighter. The question is at what rate it's being produced. When you burn sugar, you don't really produce a lot of carbon monoxide-- producing CO happens more with fast burns of carbon-based flameables, like burning a lot of natural gas rapidly. Sugars and fats burn much slower than say gasoline, which means that the carbon monoxide production should be barely measurable with sensitive equipment, if at all.
At the temperatures at which the flavorings would be burning, you would have far more to fear in the way of carbon-monoxide poisoning from burning a cake in an electric oven than from a personal vaporizer-- the vaporizer would burn the flavoring at a lower temperature than the oven, and there would be far more material to produce it (a whole cake vs. a few drops of liquid).
