Vegetable glycerin (VG) is used to obtain a denser vapor and achieve a VG and PG (VPG) ratio to your liking, creating a thicker liquid.
With a higher VG ratio than PG (eg 70VG/30PG) we obtain a mixture (VPG) in which the vapor is denser and smoother in the throat.
Propylene glycol (PG) is used to obtain a less dense vapor, intensify the flavor and achieve a ratio of VG and PG (VPG) to your liking, creating a less thick liquid. With a ratio of PG higher than VG (eg 60PG/40VG) we obtain a mixture (VPG) in which the vapor is less dense and harsher in the throat.