Sorry, but if you read the terms and conditions when you originally booked, you would have seen the following or something similar:
Warranties and Fitness: You warrant that you and all other Passengers traveling in your party are physically, emotionally and otherwise fit to undertake the cruise or cruise tour; that you and they have received all medical inoculations necessary;
I copied and pasted the above from my T&C document dated November, 2019 (also attached). Similar language appears in my other pre-Covid T&C documents. None of this has anything to do with risk free - the T&Cs issued at any time gives Viking the right to require inoculations, which would include vaccinations for Covid. This is a standard requirement as outside of Covid, some countries require other inoculations.
Don’t blame Viking as the requirement was always there.
vrc_us_contract.pdf