I emailed a response to you, but thought I'd answer on the board as well. The docks that Viking was using last summer in Antwerp for the Viking Sky were Kattendijkdok and Willemsdok. The docks are about 10-15 minutes from the Centraal train station. (Use Centraal, not Berchem.) Your cruise documents should give you specific information for your cruise. Our departure information told us to have the taxi take us first to Kattendijkdok, where the ship is usually docked. If the ship was not there, we were instructed to go on to Willemsdok. They are not far apart, but you can't see one from the other.
Our taxi fare was 8 euros including tip for 3 people and 6 pieces of luggage.