I travel 90% of the year and there is no reasonable financial incentive (especially if they are just flight vouchers) they could offer that would make me volunteer my seat. If I'm travelling, it's for business and I have no desire to be late. If it's travelling back home, I want to see my family and I'm not going to miss that flight if I don't have to. On busy commuter flights, I would imagine most passengers share my sentiment. This would ultimately lead us into a situation where someone is randomly removed from the aircraft against their will.
Furthermore, being offered flight vouchers to give up your seat is really stupid on commuter flights. Pretty much every person on those flights has more miles than they know what to do with. Offer cash instead.