For what it is worth many of us hear have experienced a cough when we have made the switch. The airways are lined with tiny little hairs called cilia and smoking puts them to sleep. So when we stop smoking they wake up, vaping just doesn't cause this much of a cough. So that is what I think is happening, so of course when you stopped vaping and started smoking the cilia went back to sleep. I am surprised that your GP didn't tell you this but just seems to have placated your symptoms with a cough supressing liquid? I am sure if you do a quick google that you will find that a cough is quite normal when you stop smoking as we have been damaging our airways for years and gunking up our lungs with tar. It is really common. I obviously cannot comment on your other health issues.