I most likely will not sail Viking unless it is a last minute trip as I won't pay for a cruise a year in advance or pay a significantly higher deposit compared to other river cruise lines.
I don't know if you are new to river cruising but there are a lot of lines that do river cruises (and do them well). Some are ranked higher than Viking, some are equal and some are less. Almost all the other lines have better deposits and final payment schedules than Viking.
Viking has name recognition that is incredibly impressive. Not sure if you are fully committed to Viking but it would be worth looking at Amawaterways, Avalon, Uniworld, Emerald, Scenic, Tauck and a few others I forgot to mention. Compare what is included and what isn't with cost. You might be surprised that some higher lines are priced similar to Viking.
Good luck!