American Airlines and Oneworld partners Cathay Pacific and Japan Airlines have Los Angeles LAX to Ho Chi Minh City SGN, Vietnam around $510 to 540 round trip for flights March to May and September to November.
Prices are lower for AA-JAL flights, however, many of these tickets priced a couple hundred dollars more than shown on Google Flights. I had best luck with AA-Cathay Pacific for spring 2017 and AA/JL for fall 2017 dates.
American Airlines / Japan Airlines
Los Angeles LA – Tokyo NRT – Ho Chi Minh City SGN – NRT – LAX
American Airlines / Cathay Pacific
Los Angeles LA – Hong Kong HKG – Ho Chi Minh City SGN – HKG – LAX