Are you looking at tickets for purchase on the American Express website or for award tickets on airline websites? Many of the people here are referring to award availability on different airline websites, but it sounds like you're looking for tickets on the Amex site.

Once you find availability on an airline's website, you can transfer points from AME to that partner and then book at that price. That will be cheaper than buying a cash ticket, but availability is much more difficult to come by on flights to Asia. You may want to try and find availability from JFK, EWR, and PHL to anywhere in Asia and then pay for a cash ticket from that Asian airport to SGN.