VG is practically always made of veg. oils, like palm oil or soybean oil or rapeseed oil or a combination etc. If raw ingredients used instead, then I believe the oil still has to be extracted first anyway, since glycerol(which is the +99% component of usp glycerin) is a 1:4 part of every triglyceride in the fat of the oil and not found anywhere else. However, as Alien Traveler stated, getting the purity up from the produced crude glycerin is the real challenge here for simple home setups I believe...
Still, I find it fascinating to think about the possibility of making ones own nicotine and vg, from standard materials like veg. oils and rolling-tobacco, so as to be entirely self-sufficient and also not relying either on any fda restrictions whatsoever

Not that vg will ever be banned/over-taxed, but nonetheless...