This is a major difference between airlines... I'm in the process of moving to Delta from Southwest after the RR2.0 switch. I'm at 29 segments after this trip I'm on now (since February), and thus far have never been on one where volunteers were needed.
With Southwest, I was very successful in getting VDB compensation on a regular flight from Denver to Kansas City (Fridays at 1:50 p.m.). I'd volunteer every two weeks when I was there, and was successful in being bumped one time out of four.
There were several times in the past two years where I volunteered, was "paid", and then was able to actually board the same flight because the person who had checked in didn't make the flight. Never was I ever asked for the voucher back.
I asked once why sometimes I get paid right away and other times I get paid only after the flight leaves... Her response = we can see (and wouldn't pay early for) an internet checkin, and differentiate it from airport checkin or one where bags have been checked. Both of those situations would cause a GA to be more likely to issue VDB before the door closed.
In any event, obviously I'm still learning the art of Delta, because I'd have stayed in the gate area to see....
