We'll be renting a car in Innsbruck, driving to Fussen, Berchtesgaten and Hallstatt, and dropping dropping the car off in Salzburg. I've seen references to needing a pass, or "vignette" in Austria and possibly Germany. What is this and where do you get one?
Renting in Austria, the car should already have one. Ask your rental company when you arrive.
No vignette needed for Germany. As the other poster mentioned, if you rent the car in Austria, it should come with one. If you drive into Switzerland, however, you will need a separate vignette, which you can buy at the border.