The Austrian Air nonstop from Vienna to Krakow doesn't seem to fly on weekdays out of the summer, and I don't want to fly Ryan Air. Hence it seems train is the choice. What are people's suggestions about which train to take and through which platform to buy it. This would be for September 30.
I would buy the tickets from ÖBB; www.oebb.at
A short overview of the options: https://www.seat61.com/international-trains/trains-from-Vienna.htm#Vienna-Krakow
I did exactly what Badger recommends.
I also checked and Austrian Air has two nonstop flights that day between VIE and KRK.
Google flights is showing that Austrian has two direct flights that day, both for $119 one way.
Maybe look harder at flights?