How long did you smoke before you started to vape? If you were a long time smoker, then perhaps your are suffering from healing pains. Damaged lungs can and will regenerate after you no longer smoke but this takes time.. Possibly the smoke damaged tissue and air sacks are at a tender stage and are simply getting irritated by the PG or just from expansion from big vape hits. Like when you bump an injury that is healing, it hurts. If this is the case, given time and more healing, the sensation should disappear
I am not a doctor, nor have training in health care, except for my first aid merit badge that I received in the 60's. I have suffered broken ribs that punctured and collapsed my lung, and your symptoms are very similar to what I experienced upon recovery from those injuries.
One of the things the doctor gave me was the inhalation device to exercise and expand my lungs to regain my full lung capacity, had a ping pall ball in a tube, so that when you inhaled on it the ball would float up, there was a scale on the side and the goal was to get the ball up to the top and float there for long as possible. Not fun and this did cause chest pains as you described and did pass with time.
Then again this might not be what's happening, but it could be. Just wanted to suggest a possible positive theory.