You only get the DVC discount if you have your DVC card with you when you go to Guest Relations. This applies whether you have purchased the vouchers in advance or are buying the APs on the spot.
Both times I've done this I fortunately had my DVC card in my wallet, but had completely forgotten that I would need it!
I'm not sure what they do to you if you purchased DVC-discounted vouchers but don't have your DVC card with you when you (try to) exchange them, but I think it involves flogging.