Viking and every other cruise line doesn’t want to sail with empty cabins. It’s a wasted fixed asset. If DVs and Vs are filled and they have a waiting list, they probably offer upgrades to existing paid customers to free up lower priced and easier to sell cabins. How they choose customers to offer upgrades is anyone’s guess. It may be past customer history, but I was on a river cruise that offered an Explorer Suite upgrade to a first time cruiser. As I said, it’s one of the great mysteries of the universe.
To the OP, Viking doesn’t negotiate the price. If there is a “last minute sale,” it’s advertised via email, direct mail, social media, or other marketing channel. That discount code is available for anyone to use.