The Reds not only lost Tuesday night's playoff to enter the Elite tournament, but also missed a chance to bank $1.1 million in prize money.
Instead, Cong An Ha Noi took all the spoils, courtesy of goals from David Henen and Brayan Perea at Adelaide's Coopers Stadium.
Adelaide are now tipped into the second-tier ACL Two competition, leaving fellow A-League Men's outfit Newcastle Jets as Australia's sole representative in the Elite.
The Jets earnt direct passage to the top-tier tournament by topping the A-League Men's table at the end of the regular season.
Adelaide finished second, setting up their playoff encounter against the V-League champions whose line-up included former Reds captain Stefan Mauk.
Adelaide collectively couldn't control Belgian-born Togo international Henen, who orchestrated a stirring victory for the Vietnamese outfit.
"I don't think it's a big setback," said Reds coach Airton Andrioli.
"We're in Asia, we're obviously happy for that.
"We were doing everything we could to maybe go into that Elite competition, but it was not meant to be.
"It was a tough game - we played against a tough opposition that played the game really well."
In their first outing in Asian competition since 2017, the Reds enjoyed the better of the opening 40 minutes.
But Adelaide failed to fashion genuine chances after Juan Muniz went close nine minutes in with a curling 30 metre free kick that drifted wide.
And in the 41st minute, the visitors struck when Adelaide's Panagiotis Kikianis found himself in a tangle with influential attacker Henen.
The 30-year-old and Kikianis collided in battle for a through ball and the Reds defender fell, leaving Henen unmarked.
One-on-one with Adelaide goalkeeper Josh Smits, Henen coolly drove a low left-footed shot into the net.
Henen put an exclamation mark on his display in the 64th minute when he turned provider for Colombian-born striker Perea.
Henen found space along the right flank and his canny cut-back found Perea, who expertly angled a first-time strike for the visitors' second goal.
Before a 6,451 strong crowd featuring a large contingent of Adelaide's Vietnamese community, the Reds couldn't muster a reply.
Adelaide midfielder Fabian Talladira fired a left-footed attempt wide in the 72nd minute and, a dozen minutes later, midfielder Ryan White launched a speculative long-range shot high of the target.
A minute into extra time, White had a golden chance denied. Unmarked and two metres out, his header was deflected into the post off the right glove of goalkeeper Nguyen Filip.
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