I will be driving from Rothenburg down to St Gallen, and on the way back stopping in Vaduz, Bregenz, and then heading over to Fussen & Neuschwanstein Castle. It looks like my route to St Gallen will take me down A7. Is there anywhere that I shouldn't miss along this drive?
Noerdlingen, another old walled town, less touristy than Rothenburg, with an old church in the center. Sits in an ancient asteroid impact crater.
Ulm, with another church with the worlds tallest bell tower.
Be sure to get your vignette for the car entering Switzerland for 40 CHF.
No veering off from the main route, then Lindau.
If St Gallen and its surroundings is your only destination in Switzerland, I wouldn't bother with the vignette. Avoiding highway 1, you don't lose that much time getting to St Gallen from Lindau.