I can't say for sure, not having done a study on the subject, but I'll draw on my nursing knowledge to answer as best I can.
To answer only your subject line: no idea. It might be. Certainly it can dry out your lungs, which could cause issues if you don't stay hydrated. But as confident as I am about vaping being "safer", I have no evidence to back up jack squat.
I wouldn't worry about pneumonia though. Technically, pneumonia isn't caused by fluid in the lungs. Too much fluid in the lungs could provide a nice breeding ground for bacteria, thus making you much more likely to develop pneumonia, or contaminated fluid could definitely cause an infection. E-juice, probably not so much. PG acts as a mild antiseptic, and condensed vapor probably isn't going to make much fluid in the first place. VG however, as well as any sugar based sweetener, could feasibly provide nutrition for bacteria, so I suppose it's possible to increase your risk. I just doubt it. Again, though, no evidence based research can back me up on that, so remember that this is only my opinion.
So my best guess says don't worry too much about pneumonia, but there could be other harmful effects I'm unaware of.