Restaurant ideas

In Vienna we have been multiple times to Lugeck near St Stephan's church for great traditional schnitzel and to Steirereck in the Stadtpark for amazing breakfasts and lunches.

In Salzburg we have enjoyed Zwettlers.

In Vienna .... and to Steirereck in the Stadtpark for amazing breakfasts and lunches.

Although they are both part of the same enterprise and on either side of the same building you must distinguish:
Meierei im Stadtpark - excellent breakfast and lunch (*)
Steirereck - Austria's #1 restaurant (3 Michelin stars), price per person €100+ for lunch and €200+ for dinner

(*) in my opinion the original Wiener Schnitzel (veal) is better in the Meierei than in the above mentioned Lugeck.

Weissenkirchen /Wachau:

I don't know your food preferences, but if you like fish, you should consider this renowned restaurant in Weißenkirchen: https://www.heinzle.at/
It is said to be the best fish restaurant in this area. Note that it is closed on Mondays and Tuesdays, and on Sundays it is open only at lunchtime.

An alternative you can find here: https://www.kirchenwirt-wachau.at/en/enjoy

Vienna:

This is Vienna's most comprehensive restaurant guide. Although in German it is easily understandable. You can select features like "Empfohlen" (recommended) then you get only the better places, or "Bezirk" (city borough), or type of cuisine.

https://www.falter.at/lokalfuehrer?r=Wien

If you want to delve into Vienna's most popular street food, you should watch this video.
https://www.youtube.com/watch?v=2jMdLfZ7sDk

Another restaurant in Salzburg that we loved was Johaneskeller.