They can't scan the voucher at the parking lots. Some people have reported being allowed to park after showing the voucher. If you have to pay, you can still take your receipt and get a refund at the ticket booth when you activate the AP.
I purchased my AP and paid in full prior to my visit, so they actually gave me cash when I showed them my receipt from parking that day, so if you wanted to by your AP prior, that's an option.
I wonder if this works the same when picking up the pass at will call with just the code? Or is it easier to just go to DTD to park and pick up the pass...