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The other major hub that normally serves Vietnam is Bangkok (I wouldn't recommend flying to ICN or NRT from Brisbane). I found for my flights Bangkok is slightly more expensive to fly to, but overnight lodging is less expensive than Singapore.
I have seen flights from Vietnam to Phnom Penh and Siam Reap (I assume there are flights in the opposite direction), but I suspect they will be more expensive than Bangkok due to a smaller supply of flights.
If you stay the night then connecting through Singapore or Bangkok are probably about the same. If you only have a brief layover I would go through Singapore.