I've tried and tried finding sources of Vegetable Glycerin, in other words, how it is made, and what ingredients.
Apparantly the sources can come from various plant forms, some say 'vegetable', others say 'made from palm oil' etc. etc.
so am thinking that some persons can have sensitivities to various things that would make VG unusable. Perhaps allergies to nuts (palm oil) etc. I know that would spur mucous over-production, sinuses, stomach, bowel, liver, etc. It may be that different brands may be acceptable, some not.
Do not have more time for searching right now, but perhaps someone may be able to come up with alternatives. I definitely cannot use PG, but my VG doesn't seem to bother me, as of now after a couple months of use.
Sometimes this can be a cummulative problem, ok at first into full blown aversion to a product.
Check the DIY threads, sometimes there's a good idea put forth!