As others have indicated, V is the inventory bucket for complimentary domestic FC upgrades. There are others (X, G, and Z) that DL uses as well for different purposes. Once the upgrade window opens, DL assigns any available V inventory to elites who are eligible to clear an upgrade at that time. It's possible there are more elites than upgrade inventory, in which case V will get depleted by those with the highest priority and the rest will either have to wait for more V to be released or go to the battlefield. It's also possible that there are fewer elites than available V inventory, in which case it will upgrade everyone who's eligible at the time and then have some V left over.

DL adjusts V inventory up and down depending on how many of the remaining FC seats it thinks it can sell. Some flights never have any V inventory available and only do upgrades at the gate. Others have plenty.

Careful when watching the seat map, seats might be sold but not assigned. If you see F6 V0, it means that DL is willing to sell 6 seats in FC, but isn't currently willing to give any of them away to elites within their upgrade windows.