The only way to collect before yougot to the airport is if UA feels that they will need many volunteers due to a) computer error, b) major human error, or c) schedule changes that remove flights on full days.

In the event of any of these situations, UA would call you, or email you, or use whatever contact info you have in your reservation to call and offer alternate flights with a usually reduced VDB compensatin. (reduced because the desparation of UA, and the inconvienence to you of being able to pre-plan is lessened.)

Flights are oversold all summer long. If the upgrade window for 500 milers has already passed, they have opened up Y seats by upgrading those ahead of you into F.

Bumps happen, but planning on them is worse than playing roulette. The deck is stacked becuase the airline holds all of the data. There are days when the bump seeker wins, but more often then not, they get the service they paid for, and no bumps are required.

In any case, show up a bit early, check in at a counter, and ask to be added to the VDB list, even if they don't prompt you.

Don't insist on travel credits vs free ticket, as many posts have pointed out, the airlines may "de-select" you in favor of someone willing to take the free ticket if offered.

Good luck