I presume it is the flute Wikipedia describes as
The sáo (also called sáo trúc or "sow trook", pronounced [ʂǎːw ʈʂǔkp]) is a small flute found in Vietnam that is traditionally thought to contain the culture and spirit of Vietnam's countryside.
That explains the instrument but not the two note motif. I assume the particluar interval between notes is common to Vietnamese music, but why do i know that? How did this auditory shorthand get to American culture so that i can hear two notes and know "Ah, this has to do with Vietnam"?