Goals from Brisbane Roar duo James Donachie and Jamie Maclaren sealed the Group D win for Aurelio Vidmar's side.
After a 1-0 loss to the United Arab Emirates, the Olyroos needed a boost and took just two minutes to open the scoring.
Vietnam couldn't clear the danger from a corner with the ball landing at the feet of defender Donachie to smash the ball home from just outside the six-yard box.
After the break an almighty scramble from another corner saw a Mustafa Amini effort well saved by Minh Long before two failed clearances ended up in Andrew Hoole teeing up Maclaren for a tap in from close range.
In the group's other game, Jordan and UAE played out a goalless draw to take them to four points.
Australia face Jordan in their final group game on Thursday morning (3.30am AEDT) with the top two from the group qualifying for the quarter-finals.
Olyroos Lineup: 1. Jack Duncan (gk), 2. Jason Geria, 4. James Donachie, 7. Andrew Hoole (17. Jaushua Sotirio 66’), 8. Ryan Edwards, 9. Jamie Maclaren, 10. Mustafa Amini (14. Terry Antonis 86’), 19. Brandon O’Neill, 20. Alex Gersbach, 21. Brandon Borrello, 23. Thomas Deng (3. Giancarlo Gallifuoco 78’)
Unused Substitutes: 5. Cameron Burgess, 6. Josh Brillante, 11. Connor Pain, 12. Aaron Lennox (gk), 13. Stefan Mauk, 15. Scott Galloway, 16. Steven Ugarkovic, 18. John Hall (gk), 22. Adam Taggart
