VG/PG can only attract 40% of it original volume in moisture in the best situation... so... 5 ml of VG or PG can only attract 2 ml of moisture... aka water. 2 ml of water is a far streach from 1000 ml of water that need to be lost for an average person to become dehydrated.... eating salty popcorn will make your mouth feel dry too but you dont see vapor being exhaled while eating popcorn...
when some one can prove that VG/PG causes that much fluid to be lost while vaping, then I just might think about the dehydration myth being a fact... but till then its just a myth...
Vaping does cause dehydration as any experienced vaper can tell you, but that dehydration is primarily in the mouth. It does not cause clinical dehydration affecting the entire body and all its organs and systems. Vaping does not cause fingers and toes to shrivel. Nor does it cause the liver, the kidneys, the intestines, etc. to dry up.
The volume of water you cite would be needed if one were clinically, severely dehydrated, not the localized albeit uncomfortable condition many experience with vaping.
Too much popcorn can make one thirsty due to the salt content, but will not cause whole-body dehydration.
And yes, you are correct. Popcorn, while making one thirsty, does not cause vapor. Why? Because popcorn is not doused in PG/VG, heated to a certain temp and the resultant vapor exhaled.
I use the humble Protank 3 mini, single coil, 1.8 ohm, 7-8W. Clearly I'm not spewing huge clouds, but a 2-3 second puff will give me fairly respectable vapor, far beyond the capability of my tiny coil, even allowing for slightly increased air flow (I tend to keep it mid-range).
Where does that volume of vapor come from? From the moisture in my mouth. I believe the term for PG's propensity for moisture is hydrophilic (can't multitask on this crappy little phone to check).
The reason for the need for large amounts of water is that you only have that water in your mouth a short time - seconds - then it's swallowed. Repeated applications of water are needed to relieve mouth dryness. All that extra water that's swallowed does not go to rehydrating the whole body. It ends up in the toilet.
ETA: Just checked. PG is hygrophilic. ;-)