Glycerin can be produced from vegetal or animal origin.
The #1 reason why people prefer to use vegetable glycerin (from vegetal origin) is to avoid BSE (Bovine Spongiform Encephalopathy) or TSE (Transmissible Spongiform Encephalopathies). Both very nasty diseases (one of them best known as mad cow disease).
USP grade chemicals are usually tested for nasty
dna bearing organisms. The problem with these two diseases, is that the culprit is not a bacteria or a virus (no
dna, just a protein).
In a nutshell: VG is good 100% of the time. Animal glycerin: who knows? There is no good test.
Try to stick with VG if you can.