Without checking prices, I will throw this out

KUL -> HKT -> (BKK->SGN) -> REP -> KUL
KUL -> HKT -> (KUL->SGN) -> REP -> KUL

I want to say it's cheaper to fly to Siem Reap from Vietnam or Kuala Lumpur. Bangkok to Siem Reap is expensive due to a monopoly.

And I don't think you can fly from Phuket to Vietnam directly.