in general, United uses ACARS for everything that happens once the brakes are released up to and including when the brakes are set. During irrops when a flight goes to "decision" or has a customer service or ramp issue, they are generally good up until the last 15-20 minutes about having flight status updated. It is all tied into FLIFO for their system.

For VX, I have noted since day one that their web based Flight Status system isn't worth the pixels it's printed on and simply don't use it for anything. I think their internal systems used by reservations are more reliable in this regard.