There are no direct commercial flights from Brisbane to Vietnam.
It seems like connecting through Singapore is the most common routing.
However, there are other routes.
Any suggestions? What are the considerations for common, reasonable routes?
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.