I was also on this holiday last year - Hi Hils! It was the proverbial curate's egg - good in parts. I agree the group was too large - ages to get on and off the coach, loo queues in the cold, delays in service for meals, didn't really gel as a group and I don't think I managed to speak to everyone by the end.
The main issue with Vienna, apart from the hotel being out in the sticks, (the Belvedere was a nearby bonus!), was that the two walking tours were just that - walking tours - we didn't go inside anywhere, and places were open, in particular the cathedral - you would have thought this famous cathedral was worth a peek inside at Christmas! We passed other groups going into the Hofburg, and coming out from a daytime performance by the Vienna boys choir. The group was divided in two for the second walking tour - the group I went with duplicated much of the previous day's tour and we didn't even get within 200 yards of the Cathedral.
We did have free time - one group of girls had a horse drawn carriage ride - as it was Christmas the traffic in Vienna was much quieter than normal - this is a lovely way to see the city and if a few of you club together, not too expensive. I visited the Habsburg Crypt in the Capuchin Church - fascinating, also Demels cafe - you just have to visit at least one of Vienna's famous coffee shops! Don't forget to request pensioner rates on any entrance fees if you are "of a certain age" - always worth a try!
We were lucky with the weather in Vienna, cold and crisp, occasionally sunny. When we went to Schonbrunn I took the opportunity to visit the zoo rather than spend the time in the extensive Christmas market. The concert at the Kursalon was, as ever, very enjoyable.
Coach journey to Heviz/Lake Balaton was broken with a visit to a different town than on your itinerary - but the coffee stop chocolate factory was closed! The scenery around Lake Balaton was beautiful - we did have a couple of crisp, sunny days. Budapest was fascinating and deserves a proper visit when it is not quite so cold - it was freeeeezing! So wrap up well! I actually appreciated the free time and would have stayed longer. We were on the Pest side of the Danube, after the earlier coach and walking tour of Buda. Apart from the streets of Christmas Markets, there were several large churches/basilica, interesting bridges, the Parliament building and the moving sculpture of shoes by the Danube. I recommend the DK Top 10 Budapest travel guide.
All in all this was a promising holiday that needed the Vienna hotel to be more central and the itinerary tweaking - not helped by the awful "coach cough" which affected just about everyone by the end of our trip.
Like Hils I have exhausted the Christmas/New Year trips offered by JY and TS (apart from Malta, which I visited in the summer, and Costa Brava, which doesn't appeal for some reason!). I much prefer to be away for the whole of the "festive period" and haven't yet booked anything for this year!