There where some talk about this recently I remember; if 'Now' vg is usp, and I stated to the OP to mail the company to be sure and he replied that he got a mail from them stating that it was usp...
Ahh, here it is:
Vegetable Glycerine Question | E-Cigarette Forum
Btw, as long as the product isn't called 'Glycerin, usp', or with the usp directly underneath, then it's actually not following the usp protocol, since they also have strict enforcement upon the label etc.
Edit: Removed wrong paragraph...