My brother tried to apply a VDB voucher and a customer service voucher to a DL ticket he was buying and the website wouldn't allow it so he had to call in. What's especially frustrating is it was his first VDB and I was explaining how great it was to take bumps with Delta as unlike United or American you didn't have to go to the airport to use the voucher because you could do it all online! Figures DL.dumb would make a liar out of me.