The fact remains that most of the nicotine used in any products - pharmaceuticals, lab supplies - comes from China and India. A lot of the nicotine base used in "American made" juices is from there. Is that a bad thing?
Now, this is an interesting dilemma. To use a Chinese product sourced from the same reliable supply chain used by US pharma industry that theoretically meets USP classification criteria, but it's ... from China; or to use something extracted in a lab set up more or less ad-hoc by a more or less well established US based small (read very small) business. On the other hand, you could have in your American made juice, possibly, lower grade, non-USP nicotine imported through shady channels and you will never know. There's no way to know.
I don't know the answer.
ETA: for correctness - when you buy your 100 mg nic or whatever, that is no longer a USP grade liquid even if the 99% used was, unless the supplier who diluted it meets the criteria for USP.