We have done several tours with Tauck. There are at least 3 occasions when a sport coat may be appropriate: the welcome reception dinner, the special event onshore and the farewell reception dinner.
I remember once checking into the hotel in Munich on our first day and meeting a couple who were also on the tour. They had just seen the official itinerary and the husband decided that he really needed a sport coat after all.
So they left the hotel, walked downtown, and bought one for him!