NZ police would not disclose why efforts to return the men had stalled, but the two countries don't have an extradition treaty so Hanoi would have had to turn over the alleged attackers voluntarily.
"It's disappointing that police requests for extradition have been unsuccessful," a spokesperson for NZ Minister Chris Luxon said in a statement.
"Clearly it is not the outcome we wanted."
One of the women said she and her co-worker were attacked at the Vietnamese restaurant where they worked in Wellington in March 2024, days before Vietnamese Prime Minister Phạm Minh Chính was due to visit NZ.
The accused men were "associated with the police" in Vietnam and had met with officers at the police training college near Wellington, Luxon said when the episode emerged last December.
The woman, Ali Cook, told The Associated Press that she and the other server were attacked in a private karaoke room by two men who pulled them into their laps, pinned them against a wall and groped them.
She said she was forced to drink alcohol and believed she was also drugged.
Cook, a US national who was then 19, said she sustained an injury in the assault. The women reported the attack to the authorities the next day.
The Associated Press does not usually identify people who say they were subjected to sexual abuse, but Cook said she preferred to have her name used.
Investigators had "no doubt" the women were indecently assaulted and would have brought charges if the men had remained in Wellington, Detective Inspector John Van Den Heuvel said in December.
Indecent assault is a legal term in NZ covering unwanted physical contact of a sexual nature. It is punishable by up to seven years in jail.
NZ police said in February that a senior investigator had met with counterparts in Hanoi as Wellington sought to have the men extradited.
Cook told the AP that police officers couldn't explain to her this week why extradition attempts had failed.
"I really do feel betrayed by the government," she said.
She added Luxon had visited Vietnam in February to upgrade the countries' relationship to a comprehensive strategic partnership, which suggested diplomatic channels were otherwise productive.
Investigators weren't following further leads but would reopen the file if either of the men returned to NZ, Police Superintendent Corrie Parnell said.
"We know this is disappointing for the complainants in this case," Parnell said. "We can assure the complainants we are still considering all possible options."
NZ's foreign ministry has "conveyed its serious concerns about the matter" to Vietnam's government, a spokesperson said.
Neither Vietnam's foreign ministry nor the Vietnamese embassy in Wellington responded immediately to a request for comment.
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