While my success rate with VDBs is very low, I do recall one experience where my VDB was successfully completed before boarding:

After flying SEA-MSP on a Friday afternoon, I checked my 1830 connection status to ORD - it was on-time, but what caught my eye was that the MSP-ORD 1700 flight had been cancelled ...

DING DING DING: I immediately realized that the cancelled flight would b a prime flight chock full of "very important" businessmen (ie full Y tickets). I proceeded immediately to my gate to volunteer, where I found pandemonium -- all the passengers booked on my sold out 1830 flight plus nearly all the passengers from the cancelled 1700 flight (including hundreds of guys in business suits: DING DING DING).

I asked the gate agent if they were seeking volunteers; she replied yes ... and it appeared she had not even solicited volunteers yet or begun a list. She took my name, then made an announcement for volunteers. Ten minutes later the GA called me back to the podium and fully completed the VDB process: issued vouchers, and rebooked in F on the next MSP-ORD flight leaving about 1:30 after my ticketed flight.

This was before any boarding had begun - in fact, I think it was well before any boarding got underway given the level of confusion at the gate. (I ran away to the Club with my vouchers at this point should anything jeapordize my VDB).

My new flight departed on schedule, while I suspect my original flight pushed back 30 to 60 minutes late, so I arrived in ORD about 45 minutes later than my original flight.