If you’re in Vienna, then I recommend making the trip and crossing the Danube River to go to Bratislava, capital of Slovakia. It’s quite remarkable that two capital cities are that so close to each other but this is due to certain events in history as the Bratislava did not have a lot of prominence because of Vienna and to a lesser extent, Budapest, both capitals of Austria and Hungary, respectively. At the end of World War I, Czechoslovakia decided to break away from the Austria-Hungary Empire and some of its legions decided to take Pressburg (the area that it was called back then). After annexation of a German/Hungarian-majority city, it was then called Bratislava which then Czechs and Slovaks started to move into the city while the Germans and Hungarians fled. Eventually, after it was said and done, the Czechs and Slovaks decided to divorce for themselves so Slovaks decided on their largest city to become the capital since Prague was the capital of Czechoslovakia but it was in Czechia.
In all honestly, there is not much to see in this city as Vienna took most of the importance away (as did Prague being the capital of Czechoslovakia) and lastly, Bratislava was bombed and artillery barraged during World War II. It also did not help that when the Soviets captured the city from the Nazis, the Red Army did not do much with the city or country either afterwards during the Communist era. Instead, the communist Soviet bloc basically let it sit around until both the Czechs and Slovaks broke off their communist chains and became independent of communism and of each other.
So if you want to visit and see another country then I recommend making an easy day trip from Vienna to Bratislava. An interesting thing to note is that if you ask the locals if they are old enough to remember the Soviet/Communist days, they said that it was easier to be bilingual in both Czech and Slovak because for example, TV shows had subtitles in the other language if one was being spoken. Nowadays, the younger people have a hard time understanding each other that they sometimes revert to English to communicate with each other. Surprisingly, the Slovaks are pretty good at English as are the Czechs.