I'm pretty sure most "vape store" vg doen't include pg, but I defenetelly hear you on the viscosity, and hence im also myself quite sure that many of those vg's are pre-thinned with distilled water even though they many times don't bother to state so on the bottle or description. Some good stores e.g. clearly state that they have "aqueous vg" with between 5-20% water to make for a better vape experience, and they sometimes also have the straight vg unthinned.
No matter what veg source vg comes from, then if it is usp or other pharma-grade(ep/bp/jp), then there is allowed a range of viscosities, which accomodates for the allowed water allowance; up to 5%. That and how warm it currently is, defines the viscosity, which is about 1.24x to 1.26x when 20/25C. Also, I suspect some unthinned brands, to also have accumilated water from the atmosphere additionally, since vg is highly hygroscopic meaning ratains water greatly from any source...
Like you, I have noticed how adding water makes for a much hotter vape and at higher wattages then it simply gets too hot. That is because water greatly lowers the boiling point of the vg, so the evaporation or boiling is occuring at a much enhanced rate than usually...
