Train from Vienna to Prague is 4.75 hours, one way, that is quite a lot of travel for a day trip, leaving you very little time to see Prague, before needing to turn around make your way back. Also, with a flight out of Vienna on the day, this seems like an ill-advised plan. Here are a couple of past discussions on the subject of making a day trip to Prague from Vienna if interested.
Bratislava is considerably closer, about one hour by train. You can cover a majority of the better attractions there as part of a day trip. Why not just do this instead on your last day and save Prague for another trip. Here is a trip list of good attractions in Bratislava if helpful.
Weekend in Bratislava (what you can see/do)
Hope this is helpful and that you enjoy your time in Central Europe.
It is not advisable to visit Prague for just a day, it is feasible though. Buses of WESTBUS depart from the Vienna International Bus Terminal in Erdberg VIB (subway stop: Erdberg) while buses of STUDENTAGENCY leave from Praterstern Interchange (subway lines U2 and U1). Buses to Bratislava also leave from the VIB.
I don't think it is realistic to go to Prague for the day and be back at Vienna airport by 8pm for a flight at 10pm. Bratislava is a better bet as you can get to the airport directly from Bratislava in an hour or so.
I agree with others that Bratislava is a better choice as you can spend more time without worrying about time. You can cover main parts of Old Town easily.
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