I don't think you, specifically you personally, should book travel directly on your own. You would be better off using a travel agent who understands how vouchers and insurance work, what the cruise line vaccination policy is, and with the time to explain it to you before you spend money.
Your insurance does not have "$0 of coverage". It has coverage for medical emergencies, lost luggage, a whole range of covered losses, in addition to your actual cruise fare costs - which is indeed $0 in this case.
The T&C of vouchers are not secret, Viking's Vaccination policy is not secret. Within the last 2 weeks there was another thread almost identical to yours, right here, from a chap who accused Viking of stealing his money. There too the complainant had not read/understood vouchers or the insurance policy he had purchased and blamed others.