We just returned from an extended trip to Europe that included four days in Vienna. Here are a few of my suggestions:
-We stayed at Motel One Staatsoper. The Rick Steves guide has this as a $$ hotel, I would say it's on the more expensive side but worth it. Great location, right around the corner from the opera house and Karlsplaatz, short walk to the subway. Rooms are very nice and well appointed. Best breakfast buffet we had on the entire trip.
-My wife and I are both vegan and found Vienna to be very vegan friendly. Close to our hotel was Vegiezz, excellent food and outdoor seating. We ate there twice in four days.
-Find a small restaurant named Trzesniewski, on a side road off the Karlsplaatz. They serve little open-faced sandwiches that were amazing and a real Vienna experience. They also have a location in the Vienna airport.
-Everyone wants to have the sachertorte at Cafe Sacher. Very long lines all day long! Try Demel, which is close to the Hofsburg. Shorter lines, great pastry (out of this world apple strudel) and a old Europe atmosphere.
-If you are also going to Bratislava (highly recommended), the Twin Cities Liner is a quick and convenient way to get there and it drops you off near the city center. Take a ride on the Danube. More expensive than a train but lots of fun.
We found Vienna to be an amazing city and plan to return!