In November 2023, our cruise director said you can apply vouchers to cruises you've already booked IF you haven't made final payment yet. He said to call Viking and they will apply the voucher value toward your balance due.
Check the fine print before signing up for vouchers. If it's not explicitly spelled out the way I've described, ask your CD. If the answer is in your favor, get it in writing. Note that only one voucher per person per booking can be applied, and that seems to be consistently enforced and no negotiable.
Good luck, and enjoy your upcoming cruise!