I used the plat travel agent to book flights using the travel credit with Vietnam Airlines, which does not accept AMEX . I knew that... was curious if they could find a way, and they did. They somehow booked the flight after I provided them with my AC itinerary and booking code for flying into Asia (receipt showed something about emergency booking, Montreal QC).
A second time I booked them with them, they were quick and helpful and got the exact price I wanted after providing all info as per Matrix Airfare Search without booking fees.