Hi, I did similar trip last year at the same time and currently live in Geneva so a couple of comments...EVERYTHING in Europe is closed on Christmas and usually on the 26th as well. I would recommend that you plan on doing activities at a nice hotel or perhaps snow related like skiing ect. during this time period. Also Vienna has a lot more to see than Prague so you may want to adjust your time schedule if possible. You'll also find the weather to be extremely cold and damp this time of year so make sure to pack VERY warm clothes and boots for walking around the cities. There is tremendous history and the food and people are wonderful. Have a wonderful time!!