As you first go off stinkies, your lungs can start to recover and coughing can come back for awhile while things get cleared up. However, you appear to be getting irritation from it, so a few tips:
1. If you are using an 80/20 mix, consider trying a 60/40 or even 50/50. A small change in the ratio can go a long way.
2. Make sure you are drinking enough water. Drink more than you think you need to.
3. Try lowering your voltage, if you're getting harshness at the end of your vape that could irritate things.
4. Try modifying your technique and do mouth-to-lung inhales. This helps cool the vapor more, and lets you find your lungs threshold while they get used to it and recover from your smoking. You draw the vapor into your mouth, then open up the back of your throat and inhale as much or as little as feels OK into your lungs.
Robino1 explains it way better than I do, but I don't think they will anymore since I kept asking them to explain it slower with more details over and over, so I'll point you here:
http://www.e-cigarette-forum.com/forum/ecf-library/337017-inhalation-technique-e-cigarette.html
*gives the horns*