Just three days after smashing India 11-0, Japan were again in a dominant mood as goals from Riko Ueki, Maika Hamano, Aoba Fujino and Kiko Seike sealed the three points against Vietnam in front of 3648 fans.
Given eight of the 12 teams make the quarter-finals, world No.36 Vietnam would have gone through to the final eight with a narrow loss to Japan at HBF Park on Tuesday night.
But the size of the defeat means the Philippines, who finished third in Group A, and Uzbekistan, who finished third in Group B, go through instead.
World No.8 Japan topped Group B with a perfect three wins and a 17-0 for-and-against tally, setting up a quarter-final clash with the Philippines.
Taiwan, who beat India 3-1 on Tuesday night, finished second and will now face China in a quarter-final in Perth on Saturday.
The other two quarter-finals pit Australia against North Korea in Perth on Friday night, and South Korea against Uzbekistan on Saturday night in Sydney.
Japan enjoyed 80 per cent possession in the first half and would have led by more than 1-0 at the break if not for some strong saves from Vietnam goalkeeper Khong Thi Hang.
Ueki opened the scoring in the 21st minute with a strong header from just six yards out.
Thi Hang pulled off a stunning diving save in the 30th minute to deny a sizzling strike from Fujino.
And Thi Hang was at it again just before halftime, with her cat-like reflexes denying Ueki's flying header.
The second half was all one way traffic as Japan finally finished off their chances.
Hamano scored in the 51st minute when she latched onto Momoko Tanikawa's scuffed shot to hit the back of the net.Â
It was 3-0 in the 64th minute when Fujino unleashed a low curling strike to beat the keeper's outstretched hands.
And the demolition was complete three minutes later when substitute Seike's shot from the edge of the box hit the upright and bounced in.
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