I did some research a couple of years ago on vaping and dehydration and I found a study on the affects of nicotine on it. I wish I had the link but I don't and my Google-fu is not working tonight
I'll summarize the study results as best I can, but keep in mind it's been a while. Nicotine is a diuretic for some people and has no effect on others. I don't remember any percentages but I remember thinking the effect is probably enough to be significant for someone who already has hydration issues, but probably not for healthy folks. I believe I only found that one study on the topic, but I felt it was a high quality website and study.
My opinion on PG/VG is that the hygroscopic nature of these substances is irrelevant because even if they absorb their own weight in water, the small quantity of juice we vape makes that insignificant to a person's body hydration level. I also couldn't find anything that suggested to me that absorbing the small quantities involved could affect us chemically, but that really doesn't mean much other than I found nothing that suggest that can happen.
For me, vaping nic makes my mouth feel a bit dry which doesn't happen when I don't vape nic. I vaped 90% VG for at least a year and since about December I've been at 90% PG with the same results.