The fees for upgrading are here:
https://www.virginamerica.com/cms/vx-fees#/upgrades
It is $240 to upgrade from Main Cabin select to First on a transcon flight. VX only has 8 first class seats on each plane so it could very well be full so I would not count on being able to purchase an upgrade. I would imagine the upgrades would be more likely to be available going through Dallas. The service would be less elaborate but would also be more affordable to upgrade at least one segment.