You might have seen this before, but worth a punt if you haven't:
The safety of propylene glycol (PG) when inhaled through an electronic cigarette. Studies on the effect of the inhalation of propylene glycol through a personal vaporizer.
Personally, I suspect that vaping isn't entirely safe, but only in the same way that drinking alcohol or eating animal fat isn't safe, either. The worst case scenario, I suspect, is that there will be some specific to the practice of vaping that causes certain health problems. For instance, I notice that when using Cartomisers, esp dual coil cartomisers, the vapour can get quite hot. I seem to remember reading that certain oral cancers (lip, I think) can be triggered by repeated irritation of a single spot by a heat source, or it will be some change in the chemical when heated which mutates DNA, or some such thing. The body is complicated thing where A+B can often not lead to C but Z. Who'd have thought that by not getting enough sunlight between the ages of 6 and 16 could (so the modern data suggest) lead to MS in later life. Another example - I have a condition called central serrous retinopath, which is a busting of the fine fluid veins on the back of the eye. It is linked with elevated cortisol, but recently it became apparent that prolonged use of nasal decongestant sprays also caused it. You never would have thought that inhaling a vapour in your nose would case a problem in the fine veins right in the centre of the retina, but there you go. I'm sure there are thousands of such things, and I'm pretty sure vaping will cause one or two.
It might be that long term vaping increases the chance of chronic bronchitis (quite possible as warm dry air irritates the lungs), or increases GERD which elevates the chances of esophageal cancer. But hey, ho. We live in an imperfect world. Diesel fumes are cacinogenic. Barbecued food increases your risk of stomach cancer. Wine increases your risk of God knows how many cancers and lord know what else besides. I even read recently that oral sex increases a man's risk of throat cancer (I kid you not:
BBC NEWS | Health | Oral sex linked to throat cancer).
I suppose we just have to remember we only have a certain time, and if one thing doesn't get us, then something else will, and it will probably a result of something we willingly ingested over the long term.
Which is not to say I advocate going hell for leather with every concoction under the sun, but remember, you could live like a monk for the next fifty years, and you know what, you'd still die from something.