It depends if your
juice is PG or VG (or mixture) based. My bottles say on the bottle (the stuff from cignot has a PG or VG sticker, at least).
It won't _hurt_ to add some VG (as long as it's USP grade glycerin), but it will dilute both the mg strength and the flavor, since VG is essentially unflavored. You can add VG, however, to any PG/VG mixture from 100% PG to 100% VG.