I just finished mixing up from a bottle of GP VG. I will say that I don't think it does well with exposure. As my bottle is now half empty, I'm noticing a caramel, burnt sugar aroma just barely present. The bottle was great the first few batches I mixed, so I can only think it is the airspace in the bottle causing it.
I've kept it out of the heat and light as well. I will report later if I get any off flavors from it. Still at 4.99 for 120 ml of 36mg, I can't gripe too much.
Wondering now if I should slug it in the freezer to curtail any further degradation if it is indeed the case.