VG is Vegetable Glycerol, a safe food additive used in most of the food and also naturally exists in most of the organic food. It is sweet in taste and helps to produce more vapor.
PG is Propylene Glycol, a similar component to VG, it's used as a safe food additive and while less sweet, it makes more balanced taste.
VG is derived by extraction from plants as raw material. PG can be derived from VG, but normally as most of the organic chemicals, it's derived initially from fossil fuel (oil/gas).