I wonder, since Virgin & Alaska are on Sabre, if the upgrade process is the same as Alaska F.
"U" space must be available for first upgrades to process, otherwise you go on the standby list at the airport. There is a separate waitlist running, based on status, fare class, and time of purchase, for the upgrades into First (U) on Alaska.
As a 75K, and on Alaska, at 5 days out, if "U" is open, then I should clear for an upgrade. Golds clear at 72 hours, and MVP at 24. If U opens up inside our window, I will trump any lower status members. If at T-24 and Im still in back, I then get placed on the airport list.