Good chance with no visa for arrival to Vietnam that you may be denied boarding in SYD.
Whenever i leave Australia bound for Vietnam my passport is ALWAYS checked for a visa, i have even been called to the gate to have my passport checked for a visa.

As for short checking bags i have no experience with air asia but many airlines will refuse to short check bags to try and stop exactly what you are trying to do.

Do you have a return on the same ticket?