One of the best deals going between BKK and SGN is Air France. The always fly with half empty planes since the route is CDG-BKK-SGN-BKK-CDG and most get off in BKK.
Last time I flow it was ~US$200 OW in "near lay flat" Business Class.
Just checked Nov 13-20. Eco RT = US$237 (others 550 and up). Business RT$478 (others 700 and up). So for less than the cost of Vietnam air or Thai Airways Economy you can fly Business on AF.
Keep in mind AF does not fly that route every day.
Check prices at
Cheap Flights, Airline Tickets, Cheap Airfare & Discount Travel Deals - Kayak.com