Hmm.... I am a heavy vaper 15-20ml a day minimum, I lung hit 90% of that... If I don't consume and extra 1000ml of water a day I show pretty much all the symptoms of dehydration, I can tell right away by my urine color and it stinks quite a bit..
This did not happen when I first started with my ego, or even with something like a genitank/iClear30s it only started happening when I started to sub ohm lung hit and it is completely controllable if I drink a couple extra glasses of water per day..
There's tonnes of other factors that not may apply to everyone though, I drink a LOT of coffee I never had a problem with it before but it is a diuretic, I also have 1 or 2 beers a day usually after work again a diuretic, both of which have never dehydrated me, I always have a glass of water before my morning coffee and before I go to bed, it's been my daily ritual for at least 14-15 years.
I really like a warm vape, when it's cool it just isn't enough for me, this could be part of it after all we are pretty much holding a heating element to our faces and directly inhaling vapor off said coil so that my have something to do with it more so probably just dry mouth though. I have a feeling it has more to do with glycerin/propylene glycol, glycerin/propylene glycol are humectants, it's a type moisturizer that works by drawing water to the surface of the skin to moisturize it and it may be just a simple fact of some of us coming in way way more contact with them than others since sub-ohm'ers generally come in contact with much much more vapor than others, I used to go through 3-5ml a day no problems at all and only since I increased it has it appeared.
I'm not a scientist or a doctor please don't take what I say as fact but after extensive analysis this is the only conclusion I've come to, I've actually gone to the doctor about my dehydration because it is a symptom of diabetes luckily showed up as negative.