Well my perception is the same as yours, I must have missed the post where you qualified your statement as being perception. I have an early stage of the condition. I stopped taking the drugs after I quit smoking. However if my understanding of the disease is correct it is normal to experience significantly improved lung function after quitting smoking, and I have certainly seen that. However that doesn't mean the lung damage from COPD is reversing, or even stopping, just that the lungs you have left are working better. The amount of lung tissue affected will continue to increase. I expect, rightly or wrongly, to see my lung capacity continue to degrade. If that happens, like you I might take another look at the medications.
If I am right about that then vapers with COPD will indeed see their condition continue to worsen, despite the positive bump that comes from quitting smoking. If so, it will likely be impossible to determine statistically whether vaping has contributed to the decline in lung capacity. Even harder to determine whether one of the long list of flavor chemicals has had any impact.
Since I don't know for sure about diketones and likely never will, I balanced the odds and decided not to partake of them any more. We have to make decisions every day on subjects where the available information is incomplete or contradictory.