USP is the highest purity typically available (there's an even more pure grade, ACS/regeant, but ACS/regeant is ultra-pure grade and typically only used for high-end laboratory research & chemical mixes).
However, purity does not = taste, nor does it = "vapability".
Assuming (when I definitely knew better) that all USP VGs were basically the same, I recently purchased a gallon of
this via Amazon (to save money - we've got Amazon Prime/free shipping, and the price is dirt-cheap)...and without testing it, mixed up around 3000ml of various juices & test vials.
At first we didn't notice anything different - but after a few days I kept wanting everything I vaped to be 'stronger & sweeter' (which is really odd for me because I dislike overly sweet juices - and we'd formulated recipes that were perfect). Hey, I said to myself, it's probably just vapers tongue or our taste buds changing - I'll just add a drop or two of sweetener to the bottles and maybe a bit more flavor next time we make them (but we kept frustrating over how/why we could both end up with 'vapers tongue' at the same exact time). It was kinda frustrating, but really no big deal.
Then I answered a post and advised someone having trouble to be sure test all their base liquids without flavoring or sweetener before making any other changes...and then it hit me (argh) - we'd changed VGs and used it to mix up a bunch of juice
before we'd tested it.
So we mixed up two small batches of our base (80% VG; 9% distilled water; 9% saline solution; 2% alcohol) - using both VGs...and tested them both side-by-side. Sure 'nuff...the new one tasted 'flat', dirty, and seemed to kill the natural sweetness of the VG (which is extremely slight and was never discernible to us before). The base made with the original VG was 'alive & vibrant' with just the tiniest hint of sweetness.
:::shaking head::: We'd seen that gallon size on Amazon, that it was USP, and calculated how much money we'd save over buying what we usually bought at ECX ($8.99 for 250 ml at ECX vs $40 for a gallon on Amazon). It would be around $135 on ECX for a gallon (purchased 250 mls at a time). So we were saving a boatload, eh? :::sigh::: But the worst thing...?...
When I went to ECX to order VG, I decided to click on the Wholesale Link to see if they had larger sizes of VG. Yep, 1 gallon for $42.99 (or two gallons for $79.99)...and since shipping is free over $85 on ECX, it would have only cost me only THREE BUCKS MORE than the crap I bought on Amazon to get a gallon of what we'd been using since we started!
I recommend the VG from ECX. I do not recommend the VG that I linked to on Amazon. I also recommend mixing/tasting/vaping your bases (with and without nic) before mixing them.
I do not recommend forgetting or ignoring that basic rule of common sense and spending almost two full days mixing/blending, steeping, labeling and rebottling juices using untested VG, PG or nic-juice. 3000+mls of juice...seeya-bye.
That's my baby girl ....well, not so baby anymore, but still 
