I don't think it's a necessarily a Ponzi scheme. Sure you take money in from new reservations to fulfill old liabilities, but money is fungible. David's does not incur a loss unless 1). the owner keeps the 70% but David's does not get points back to re-rent, either because the points expire worthless (using banked points), or the owner tries to double dip; 2). the renter initiates a charge back and wins. In case 1, David's has only lost about 53% of the reservation cost since he keeps the 30% payment to owner and the commission. In case 2, he really just lost the commission, assuming he got the points back to re-rent. In all other situations, the math to both the owner and renter is eventually equal. Even if the renter made a cash reservation, the cash value is only the 70% paid to the owner. So if you can't find a discounted
DVC room, he uses whatever he paid to the owner to book a cash room. So your voucher will have value.
I read somewhere that he had 2000 affected reservations. It's hard to say what percent is in case 1 and 2. In case 1 he loses $10.15 per point (70% already paid out to owner). In case 2 if the renter does a chargeback and wins, he loses $4.50 per point. Obviously we don't know what the exact percentages are, and how many points those reservations represent. But let's assume 100 points per reservation and he take a loss on 20% of the reservations - on 10% he loses $10.15, on the other 10% he loses $4.50. His loss is around $260k. It's a blow but probably not fatal to the business. If 2.5 months of closure is 2000 reservations, he can generate over $4 million in commissions per year.
But he does have a cash flow problem immediately. He might be getting back a lot of points from owners willing to re-rent, but he may not be able to turn them into cash from future reservations, which means no commission revenue for him. Also, if he cannot re-rent the points, it can eventually become an additional loss like case 1 where the owner keeps the 70% with attached reservation. This is why he is also asking for cash back first, and re-rent points second - if he get the cash back from the owner then he has no risk of future loss if the points cannot be re-rented. This is also why he is offering the voucher instead of giving refunds - these reservation will still eventually happen, and thus there is no immediately cash outlay, and eventually he keeps the commission. If you take the voucher you are locked into making a reservation and thus generating a commission for him.
I think his voucher plan is quite reasonable. But personally I don't plan on using him again because I think the $4.50 cut he is taking is way too deep for the perceived safety to owners (or lack thereof).