We did a Viking River Christmas Market cruise in December 2022. You register a credit card when you embark. Near the end of the cruise, a tip is added to your bill. You can view the charge on the television in your stateroom. Ours was $133 per person ($16.67/day USD).
The amount can be adjusted or deleted by speaking with a person at the guest services desk. I have seen people at the desk asking to remove the tip and they did it. I guess there could be a variety of reasons - the person doesn’t tip, they want to tip individuals in person. I don’t know. I understand that tips are included in the fare for travelers booking from the UK and Australia (and maybe others).
We think the amount assessed is fair. In addition, we leave a bit more cash (local currency) in an envelope for our cabin steward(s) and sometimes one or two servers who treated us exceptionally well. We’ve done two Viking river cruises, three ocean, and one expedition. We like Viking and think the service is great. Hope you enjoy your trip!
PS - You might take a few euros to tip the local guides on excursions.