Early disembarkation?

We are taking our first river cruise in March (Viking Europe - Eastern European Odyssey) and, as always seems the case with us, have to do things a bit differently. We have managed to use our frequent flyer miles, saved for years, for first class and business class travel into Budapest and out of Vienna, but must catch a flight out of Vienna the Saturday night before disembarkation Sunday. The ship apparently arrives in Vienna from Bratislava early Saturday (there is a city tour and free time scheduled) so I don't think there will be a problem time wise, barring the river drying up or something! However, are there any protocols involved in leaving the ship that could be problematic? (Customs, etc.?) Thanks for any input.

We left a Grand Circle ship early in Germany two years ago for a personal meeting with some friends. We did not announce it to the company when we booked and told our Program Director the night before and there was absolutely no problem. They were gracious, helped us with our bags and called a taxi for us and we reciprocated with a nice tip! There are no customs formalities on river cruises as far as I know. Clarence:)