I've had MANY vouchers over the years and it does seem like they are making you jump through more hoops these days.
Last year I had two $500 travel vouchers to use but when I called the reservations line to redeem them and book a flight they basically told me that they were only good for domestic travel with a myriad of blackout dates. I was livid.
I then made a special trip to the airport and went to the airline ticket counter and they redeemed them, for an International flight, with no questions asked. I told the person at the airport counter about the issues I had when trying to redeem them over the phone and she said that I did the right thing in coming to the airport. The call center people know nothing other than what their procedures or computer tells them.
I think I'll just go to the airport next time.