My parents used Viking last year from Vienna to Amsterdam and they had a fantastic time. Here's a link of a review on the Viking Sky:
They chose Viking because most of the excursions were included. If it helps, Viking was given an award this year for best overall cruiseline for river cruising.
I have read positive reviews on the Vantage and I did read a review of Avalon and it said the rooms were smaller and service was lacking in the dining room.