Short answer Michael - If you're looking for low and slow cooked BBQ via a Cookshack or some other similar type of smoker in the cities you've mentioned, you're not going to find it.
Longer answer - I'm familiar with the cities you mention, though it's been awhile for Vienna and Salzburg. Last in Munich around 4 years ago and can't even imagine a BBQ restaurant there. Was in Budapest 2 years ago, and while the options might be a bit better, forget the low and slow que.
Of the 4 cities you're traveling to, Budapest, to me, is the most interesting because of it's ethnic diversity - an Eastern European, Western European, and Asian blend. You'll see restaurants featuring BBQ, but most will specify South African, Asian, and/or Australian BBQ. In other words, it's grilled meats. Not bad at all, but not what I think you have in mind.
One place that I recall in Budapest was known as Mongolian BBQ. A large variety of different meats were set out buffet style for your selection. Once you've made your pick of meat(s), you then carry them to a separate station for a chef to cook on a hot flat top set upon some waist high urn type device which contains the cooking medium - wood, charcoal, or whatever - I don't know. A large variety of side dishes were served at the table. They had a fixed price for unlimited food and drink. I was not paying, but I understand that it's quite reasonable. The camaraderie was great. Like a big party with a mix of mainly locals and some tourists. I understand that there are other restaurants in Budapest doing something similar, but I don't recall the names.
Enjoy your trip.