There's always the brute-force method to find V inventory:

1) Go to dl.com
2) Make sure you're logged in
3) Advanced Search for full Y fares
4) # of passengers = 1
5) Review results. "Upgrade Available"? V>=1. "Upgrade Eligible"? V=0
6) Repeat search with n+1 passengers until "Available" changes to "Eligible", n-1 = V