Both PG and VG are hygroscopic; they grab moisture from your mouth and throat in order to make visible vapor, and they will both dehydrate you, you just may not feel it as much with VG, due to its emollient properties.
Plain water is great, but any beverage without caffeine or alcohol will also work just fine. Alcohol displaces water, and thus causes dehydration on it own. Caffeine stimulates the kidneys and thus can cause dehydration from too much urination. I've found that daily coconut water helps me a great deal; not only is it excellent just for hydration, it's also high in potassium which helps counter the high-sodium diets most of us consume, and keeps the electrolytes balanced. {ETA: sports drinks are useless unless you're actually sweating a WHOLE lot, because they're far higher in sodium than potassium}
Andria