Makes a lot of sense actually, I had a rash that I was told was everything from excema to ringworm near my armpit, I stopped using deoderant(gross) and it went away. I found one product that works but sucks(axe spray) and one that doesn't. Tabac deodrant, a bit more money, but smells lovely, and I don't get the same reaction. I would suspect that it's overall exposure, and that as was said, vaping PG was the last straw. I'm allergic to petroleum from too long in factories, and its the same, a certain threshold of exposure and I become oversensitive for a time, perhaps cutting out all other PG intake, and then adding PG based vapes and flavors back over time would allow people to continue vaping without reaction. It could also be the VG, I started originally with Innovapor juices, while I didn't get any itching, I noticed my skin became extremely sensitive, and prone to reactions, similiar to dermographica or whatever it's called where you can draw on your skin with fingernails, and it will swell into the pattern, it was ridiculous! I've been vaping TV 90/10, and get occasional itching on my skin. It really depends on the flavor too I've noticed, I was much more itchy vaping atomic cinnicide, and I found that after switching to other flavors(mostly a mix of TV tobacco and geofs blend) the clear phlegm has gone away.
I'm wondering if it could be flavorings for me too, so far my rashes have subsided for the most part, though I get minor itches occasionally, I always seem to get frustrating acne for the first month or so of vaping.
If it is flavoring, perhaps avoiding inhalation to the lungs could be a solution to let you vape flavors without the reaction?
I wonder if something could be changing the PH on your skin? I get the same reactions from work if I'm not careful from the washing liquids we use(very alkaline)
Allergies are frustrating, sometimes I get an itchy throat after eating, but have never figured out what the heck causes it!
EDIT: If I get some free time I'll see if I can bug my doctor into doing allergy testing on me, I suspect a lot of it was cinnamon for me, I got acne around my mouth, when I quit vaping that, it cleared up. Might as well use the free healthcare! lol.