Although I cannot speak for EVERY manufacturer of Glycerin, USP that exists, I did personally speak to the Quality Assurance person at Humco, Inc. Humco contract packages (meaning, places their manufactured product into other company's labeled bottles) for almost every major pharmacy label in the country. From their company information they contract package for the following:
Among our wholesale customers are McKesson Healthcare Solutions, Cardinal Health and AmerisourceBergen(which is the Family Phamacy and Good Neighbor Pharmacy labels) along with most regional wholesalers. In addition, Humco supplies products to Walgreens, CVS, Rite Aid and Wal-Mart.
So 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.
Brands of Glycerin, USP not included in the above list would have to be verified as solely vegetable source by direct contact with the manufacturer/packager/distributor listed on the label.