The largely comfortable win, initiated by them scoring twice in the opening three-and-a-half minutes, meant Melbourne completed their three-match group campaign in Vietnam with 15 goals.
Their reward will be a home last-eight tie next March as they look to go one better than the inaugural continental event when they lost May's final in a penalty shoot-out to Wuhan Jiangda in China.
"Yeah, we're very excited to top the group and get a home quarter-final, it'll be unreal," enthused Golden Boot winner McNamara, who proved the difference maker once again after recovering from a slight injury knock at the weekend to lead the way.
"We're really happy to get the win and keep that clean sheet. But it was a tough match. Ho Chi Minh are a very good side, and I think we found that out in the second half, definitely."
The 22-year-old McNamara, just called up for the Matildas for the upcoming New Zealand matches, pounced in the opening minute to cash in on the hosts' defensive dithering in their own box, swooping to blast home from close range after they couldn't clear a low cross from the right flank.
Extraordinarily, before Ho Chi Minh City had barely recovered, they were guilty of more poor defending, failing to clear their lines in the fourth minute and allowing Leah Davidson to tee up a deflected low shot from Alexia Apostolakis from outside the box which squirmed through the keeper's grasp.
Already on the back foot, Ho Chi Minh City, who also had a perfect record after two matches in the group, then never looked likely to trouble last year's inaugural finalists.
City controlled proceedings and went further ahead five minutes before halftime when fine work from McNamara down the right flank set up Shelby McMahon to arrive just ahead of the keeper and deftly flick home a third.
That was goal No.15 following their club record 7-0 victory on Sunday over Stallion Laguna, from the Philippines, and a 5-0 win over Singapore's Lion City Sailors last Thursday.
To finish their fine week's work in Vietnam, there were to be no more goals after the break as coach Michael Matricciani experimented with his line-up a bit.
That included another outing for star young teen Danella Butrus, but she had to come off again near the end with an injury caused by a reckless studs-up challenge into her stomach from Phan Thi Trang, which went curiously unpunished.
But though Ho Chi Minh City were much improved after the break, City needing to defend robustly on more than a couple of occasions, substitute goalkeeper Melissa Barbieri was still rarely troubled.
"It was a lot tougher after the break," said McNamara. "Obviously, they changed some things, and maybe we need to learn how to cope with that. But no, we're just extremely happy."
