don't worry about international driver's licence.
On the return you may try to avoid Austria by driving as much as you can on the Swiss Autobahn until you see the signs for Munich/Germany. You'll avoid the A Vignette that way.

Have a good trip. There will be a border check into either Germany or Austria from Switzerland, the other way too but it is very quick and they worry more about what you carry, as far as goods are concerned. I never heard of a driver's licence check at the border and I live part time in Switzerland.