PG and VG are both Humectants, substance to keep things moist, so it is not the root cause of dry mouth but when used/vaped in large amounts it can result in a "cotton mouth" like effect. Large amounts would fit into a person using an E cig often and consistently. If you are actually dehydrated, your urine will be a very dark yellow. The lighter it is the more hydrated you are. In regards to why it makes the mouth dry is yet to be determined scientifically but based on the fact both are Humectants would lead me to think it is more of a coating build up on the tongue giving a dry feeling, thus a thought one is dehydrated. Drinking water is a good thing for many reasons and sure will aid in relieving "vape mouth".
I doubt very seriously you are dehydrated but more of experiencing the dreaded "vaper tongue". But next time you go wee, take notice of the color, if it is in fact a dark yellow it is time to load up on water.
Nicotine can cause the salivary glands to reduce secretion of saliva, again creating the dry mouth feeling. It also plays a role in the TH you get, the higher the nicotine the spicier the TH and even more affect on the secretion of saliva.
Hope this helps!
