James Donachie opened the scoring for Australia in the second minute of the match.
Vietnam had several chances to level the score but they failed to convert.
After the break, Jamie Maclaren doubled the score for Australia in the 60th minute.
Earlier, Australia fell to a 1-0 defeat to the UAE thanks to a late own goal by Giancarlo Gallifuoco at Grand Hamed Stadium, while Vietnam lost 3-1 to Jordan in the opening game.
Vietnam will meet the UAE on January 20.
A total of 16 teams qualified to play in the tournament, including Qatar, who qualified automatically as hosts.
The top three teams of the championship will qualify for the 2016 Summer Olympics men's football tournament in Brazil.
