If you don't need the flexibility and can safely commit to a specific train for the VIE-BUD sector, the MAV website will often sell tickets for much less than the fixed-train non-refundable 'Sparschiene' prices on OeBB.

A random example - RJX63 on Saturday 23rd September is showing a Best Price 'Sparschiene' ticket (this train only, non-refundable) of €38 + €3 for the seat reservation, whilst a non-refundable ticket for that specific train can be had for €20 + €1 seat reservation on MAV (First Class is still only €28 in total). Even where OeBB doesn't show a Sparschiene price, MAV will usually still sell you a non-flexible, non-refundable ticket

Conversely, Flexible tickets are often more expensive on MAV than OeBB so you should always buy these on OeBB.