My experience is this, if you are in a suite, the free vouchers show up. If you are not, then it only offers credit card bookings. At that point, when you get to the dining room at your reserved time, tell them that you want to use your voucher. They will cancel the credit card reservation and add the voucher.
The cruise I am on now had very few options for specialty restaurant availability for the week. Ship is busy, but not at capacity.