If you haven't purchased your ticket yet there's an easy solution. If you break your trip into two tickets, you'll enjoy full flexibilty and very cheap fares. The trip will take an hour longer or more, but you will not have to worry about making specific trains. If you miss your connection you can just go to lunch or so while you wait for the next train:
1. Purchase the BayernTicket (valid all day for all "S", "R", "RE" services in Bavaria) at a ticketing machine at the S-Bahn Station at Munich Airport. This ticket costs €29 total for 5 people. Get on the S8. Get off at Ostbahnhof.
2. Here get on the any train marked "RE" to Salzburg. (Previous ticket is valid to Salzburg Station.)
3. In Salzburg get on any WESTBahn (
www.westbahn.at) train (painted white with blue and green stripes.) On board you'll need to fork over €73.40 for the whole group to be taken to Vienna. The trains leave every hour or two, last departure is at 7:26pm.
Hope this provides a good flexible alternative.