See:
http://www.e-cigarette-forum.com/forum/diy-e-liquid/52669-pg-vg-labeling-purity-please-read.html
"...any Glycerin, USP labeled in any of the above mentioned Pharmacy labels (Humco, Family Pharmacy, Good Neighbor Pharmacy, CVS, Rite Aid, Walgreen's and Walmart labels) IS USP grade AND Vegetable sourced..."
I get my VG USP from Rite Aid most of the time, it's convenient, absolutely fine for the year I've been mixing. Health store glycerin is actually what I would be cautious about, unless it, too, is explicitly USP.
From the sticky I linked above: "What about all those Vegetable Glycerin products you find in health food stores? ... Kashrut(kosher) laws are Jewish religious dietary laws having to do with the preparation of food products and really have nothing to do with the labeled purity of the product....In any case USP and Kosher are 2 different things. There is no such listing in the USP for "Kosher Glycerin"...There is also no “Vegetable Glycerin” monograph in the USP....
The Dietary Supplement Health and Education Act of 1994 gives almost carte blanch license to Dietary supplement/Herbal/Vitamin/Health Food/Natural Product manufacturers to manufacture, label and market their product as they see fit(no true oversight or approval by the FDA)."