Vignette for Austria

If you know the license plate number and Country of registration you can but it online. If you don't drive the Autobahns you do not need one. The secondary roads are more scenic. Cameras record you on the Autobahns. Many Autobahns have fences, berms etc. to keep noise down nowadays, along with 100-120kph speed limits. Time allowing and vehicle type/your requirements, I would travel the secondary roads. The sticker type is the old way away. You can buy one though at the last stops on the German Autobahn. Some stretches of the Austrian Autobahn do not require one. Around Kufstein and south from Bregnenz come to mind. Smart Navi can guide you away from toll roads. Some places like the Brenner require an additional toll.
https://www.asfinag.at/en/toll/vignette/

I guess you will rent a car. Ask first whether the car has already a vignette.

Hi. Just stop at a gas station or convenience store near the border; it can be in Germany or Austria. No registration needed; about 10 Euros; good for 10 days. They will give you a sticker & it will have directions as to where to place it on the inside of your windshield. Have a great trip!

Just stop at any gas station in or after you leave Munich and buy the Austrian Vignette. It’s simple. You’ll see.

Chances are that your rental car already has one. Ask the rental car agent.