Not quite a VDB voucher, but fairly similar:

After a bad experience on an Airtran flight 4 months ago, I got a $50 discount voucher. I wanted to apply it to a flight from PNS-MDW in two weeks, but that (and all other PNS service) shifts to all-WN service and cannot be booked on FL.

Called up the representative and he told me that he's sorry, but there's nothing he can do. Has anyone had any experience being able to use such a voucher on a WN flight? It would be pointless for me to fly from MSY or ATL, as they are 4+ hours away!