Fare inventory changes so you may find a cheaper F if you check later. If it's the only VX flight and you enjoy F already, then go for the F experience. Upgrading on VX isn't as easy as with UA or other American companies; that's one of the nice parts of VX. We once flew a 4-hour flight where every seat was full except a single F seat next to my wife, which was being offered for $300 at the gate.