It's not totally water vapor, but the ingredients contain lots of water. PG is about 10% water. The flavorings are water soluble and contain various amounts of water. Most DIY mixers add another 10% to thin the mix, and your mucous membranes add more. Combine that with whatever percentage of water there is in the humid air you inhale when you
vape, there's a lot of water in the resultant exhale.
No, it's not 100% water vapor, but water is a significant percentage of the whole. I tell people it's PG/VG fog, similar to the fog machines at concerts and stage shows. The Drexel study shows that there are trace amounts of a number of chemicals in the vapor, but the left column, if I read it correctly, shows the EPA limits for normal exposure to those chemicals and we are WAY under the recommended exposures, at a tiny fractional level.
Sub ohm vapers may be exposing themselves to levels that are much higher, IMO. Huge clouds may not be the most healthy way to vape. The intent of
vaping is to get the lowest enjoyable level that removes the desire to burn
tobacco and inhale the deadly smoke particles.
The Drexel study, FWIW.
http://publichealth.drexel.edu/~/media/files/publichealth/ms08.pdf