Last Fall I was in BOS waiting for BOS-ATL-AUS and they needed volunteers for BOS-ATL. $600 and a room at the BOS Hilton were the offer and she said she would put me in F on the new flights. So I was pulled off the original flight (on which I had been upgraded) but it turned out there were no available F seats to assign on the new BOS-ATL, even though it showed F1. It's possible that something would have freed up by the morning but I wasn't going to chance it, so I said no to the deal. However, they had already assigned my original F seat. Fortunately the GA did the right thing and got another volunteer, bounced the upgrader in my seat back to Y, and put me back on my original flights, now booked in full F. So no VDB $ but got some extra MQMs as a result of the fuss.

This is mainly to illustrate the point made above: a VDB is not a done deal until you have new BPs and vouchers in your hands, so make sure they don't block your option to get what you had before until it is final.