As far as UA asking who would volunteer and for what price I suspect that they are working at improving their data on what to offer VDB. Meaning that each day they are adding loads of data points to their sample of how much VDB costs them and will allow their analytics to further improve revenue for overselling flights/routes moving forward. After all if UA can pay you $400 to VDB but can sell that seat for $401 over what you paid it's a win from revenue management. Also as is well known a significantly non zero percent of the vouchers will never be redeemed so that is another bump for the airline. At the end of the day UA will have some really great systemic data on how many people are willing to VDB at what $$ and will use that information to be more competitive in the market.