How is the language that’s spoken/written in Vienna different from standard German? If I learn some standard German, will I be able to understand, and be understood, in Vienna?
You will be understood without any problem. Actually the differences in written standard language are relatively small. There are special Austrian terms and idioms. Some of these are technically consistent with “German German”, just a lot less common. If you listen to Viennese you might have trouble understanding it, but the same would be true for a more severe dialect within Germany. As soon as they notice that you are a foreigner, they will speak Hochdeutsch to you. Perhaps you can still tell where they are from, but you will understand them easily.
Of course, they will still treat you like an autistic child, even the children. After two years of German, I logged in to a German chat line. What was being written made little sense, since they were all talking Neudeutsch. It reminded me of a Japanese TV show I used to watch about teenager girls in Japan who made fun of a visiting student from Hawaii who had been taught Japanese from her parents and grandparents. They mockingly spoke to her in the most formal Japanese and the subtitles used the archaic English terms like thou, thee, Mistress,
You will run into the same problem if you go to Bavaria, Switzerland and parts of old East Germany…some dialects are nothing more than slang and pronunciation differences - like hearing a New Yorker and someone from Alabama having a chat. However, sometimes the words and grammar can r e a l l y be different to the point of not being understood at all.
I have found, however, that most people who speak German did at least learn “high German” in school, and can understand you. The problem is, you might not be able to understand them. But after awhile, it shouldn’t be hard for you to pick up on the differences. Don’t be surprised, however, when you go to another German-speaking part of the world and they all know immediately that you were in Vienna.