I have been on the GAS tour so we did the same cities as the 8 day tour and I am going on the 8 day tour later this summer. I loved the GAS tour and so I decided to go back and do the 8 day tour.
If you are not spending extra days in Vienna then I truly don't believe you would have time to go to a concert on day 6. You will be arriving late in Vienna and you will be taking an walk with the tour guide and then dinner on your own. You may be able to squeeze it in and choose a concert that starts about 9PM. You may not want to book a concert before you go to Vienna but if you are set upon going to a concert then the guide or hotel may be able to help you buy tickets. I would suggest checking the churches that have concerts. Concerts in churches in Europe are very good and no matter where you sit in a concert you have a good seat. You are there to listen to the music and don't need the front rows.
I am just afraid that after a long day of travel to get to Vienna, the Melk stop and the , walking tour of the city and checking into the hotel and having dinner it would be tiring and also no time for a concert.
Day 7 in Vienna is the last night and the farewell dinner and that you don't want to miss.
Now Salzburg is different. Day 3 is a group dinner so again, no concert that night. But day 4 as the tour says, you could go to a concert that night. You can call the RS office and ask if the guide helps you buy a ticket or offers you an option. On the GAS tour the guide offered us an option and we paid him to buy us a tkt for a dinner concert. One tour member did not go and he bought a tkt to hear a concert at the fortress which I think would be pretty cool.
The above is my opinion and my experience being on the GAS tour and being on 15 RS tours. I hope this helps.
Have a great trip. I will be back before you go if you want to PM me about the tour.
Best,
Ann