Train Tix from Salzburg to Vienna--need to buy in advance?

You can do either but if you buy about 60 days in advance you might be able to find a reduced rate.

If buying tickets closer to departure, you'll probably find www.westbahn.at brand (private) trains to be cheaper than the national railways, www.oebb.at. Both are fast and direct on that route and don't require advance reservation or seat assignment.

You should be able to book now for cheap seats on ÖBB or FlixTrain. However, FlixTrain only runs on certain days from Vienna, if memory serves. But if you wait you should have no problem buying the day of (or day before) your journey. ÖBB has trains about every 10 minutes in the morning hours.

What is FlixTrain? This does not exist in Austria.

First, you need to know that there are two train operators for the Salzburg to Vienna route - Westbahn and OeBB. As a general rule, tickets purchased in advance will be much cheaper than those tickets purchased on the day of travel.

Westbahn tickets can be purchased in advance or on the train. Westbahn does not have a ticket counter at the train station. Here is their link - https://westbahn.at/en/

OeBB tickets can be purchased online or at the train station - NOT ON THE TRAIN. If you purchase the ticket at the train station, then it will be expensive. Here is their link - https://www.oebb.at/en/

Whatever you do, not not use RailEurope.

My mistake - I forgot FlixTrain only operates in Germany.