There is a very low open jaw fare in the $400s and $500s for United Airlines/ANA flights from several airports in the USA to several airports in Southeast Asia including Bangkok BKK, Kuala Lumpur KUL, Singapore SIN, Jakarta JKT and Manila MNL. Low open jaw fares are available from LAX, SFO, DEN, IAH, ORD and EWR. Possibly more US airports. This fare is available beginning in September. I found lots of availability in October on different routes.
I also found some low fares from Delta for some open jaw routes. This could be a fare war brewing, but already quite the deal with United/ANA ticket prices.
Visit Two Countries for Less than the Airline Ticket to One
The deal here is a United Airlines ticket like LAX to Bangkok is $629 round trip in October. But if you book an open jaw, flying United LAX to Bangkok with flight back from Kuala Lumpur to LAX, the open jaw round trip ticket price drops to $420 for the same dates. The cost to fly a regional airline one way between Bangkok BKK and Kuala Lumpur KUL is less than $50.
SecretFlying.com has listed sample city pairs and dates that work. The dates there are all two week periods. I find many date combinations work for shorter trips than two weeks. I made a few searches looking for additional cities for this deal than those listed. I did not find any other Southeast Asia airports in my limited search trying airports like Bali DPS and SGN Vietnam.
United Airlines Los Angeles LAX – Bangkok BKK / Kuala Lumpur, Malaysia KUL – LAX
$416.76 Tue Oct 11 – Wed Oct 19
ANA New York/Newark EWR – Singapore SIN / Jakarta JKT – EWR
$578.69 Oct 11 – Oct 25
Loyalty Traveler – Hotel Promotions Guide May 2016.