I have volunteered many times during the past few months to give up my seat, always to be told that the free ticket is the only option when I ask for the dollar voucher instead.
Three times I written customer service, and all 3 times the reply has said that the free tickets are preferred - but that vouchers are available upon request.
The policy handed down from above is clear, as is the almost total inability of UAL to have that policy carried out at the gate.
I have given up being VDB - and simply take my d*mn seat. It is easier than fighting with them!