If there isn't availability to Vietnam a short cash trip is probably best there.

UA doesn't fly direct to LAX from HK so you need to connect in SFO. Alternatively there are partner rates and those cost more. EVA or Asiana would be my preferred options but do cost more and involve a connection. SQ will be expensive using SQ miles or unavailable.

With a connection try KE again on the way back as an option. Start from HK vs Vietnam to avoid the additional miles from South Asia vs north.