Yes, it is possible to visit Bratislava and/or Prague from Vienna in one day, but it would be a tight schedule and involve a considerable amount of travel. Here’s some information on both options:

1. Bratislava: Bratislava, the capital of Slovakia, is located relatively close to Vienna. It takes approximately one hour to travel from Vienna to Bratislava by train or boat. This makes it feasible to visit Bratislava as a day trip from Vienna. You can explore the historic Old Town, visit landmarks like Bratislava Castle and St. Martin’s Cathedral, and enjoy the charming atmosphere along the Danube River. However, keep in mind that your time in Bratislava will be limited, and you may not have sufficient time to explore all the attractions in depth.
2. Prague: Prague, the capital of the Czech Republic, is further away from Vienna. It takes around 4 to 4.5 hours to travel from Vienna to Prague by train. While it is possible to make a day trip to Prague from Vienna, it would involve spending a significant portion of your day on transportation. You would have limited time to explore Prague’s numerous attractions, such as Prague Castle, Charles Bridge, Old Town Square, and the Astronomical Clock. It is generally recommended to allocate at least a couple of days to fully experience the beauty and cultural heritage of Prague.

If you have limited time and still want to visit both Bratislava and Prague, you might consider an overnight trip or allocating separate days for each city to have a more relaxed and immersive experience. However, if you’re determined to do it in one day, ensure that you plan your itinerary carefully, check transportation schedules in advance, and prioritize the main attractions you wish to see in each city to make the most of your time.