Hey Dalat,
Not a lot of choices as I'm looking to fly in the third week of July or early August so no saver award available on UAL. I have options of flying economy BKK-IAH using United miles and SGN-BKK using my VNA miles. Another option which is attractive at this time is SGN-ICN on I class and then ICN-LAX on O and then get an award ticket LAX-IAH with Delta or BA. Downfall for 2nd option is the 10hr layover in ICN.
I'm all ears for other tips. Cheers.