A Dutch auction for standby seats could make sense, but with VDB, there are too many variables. For one thing, if UA only takes VDB from its app, they are going to miss out on most passengers. This means you could bid $1,000 cash and still have a good chance of winning because it's just you and a handful of other people on that RJ who bothered with this app before you arrived at the gate. But if the gate agent makes announcements like they do now, where everyone hears about it, they might not need to offer quite that much for a short flight.
The biggest issue IMO is that the VDB offer is in UA dollars. Sure, try that first (since it's so cheap for the carrier), and if you get some takers but not enough, then offer cash, and keep increasing it to, say, $1,500. If that's still not enough, then IDB people and give them the same money as the VDB people plus some extra compensation because they were "volunteered" by UA.
I think the IDB compensation should be increased to 5x fare (total fare you paid, not just for that segment), minimum $1,000, no maximum. Watch IDB fall like a rock.