Dong Guangping, a former police officer turned activist, has been incommunicado since late August, leading to fears of him being deported to China.
The 64-year-old has already been jailed three times by Beijing for speaking out against the Chinese government's human rights violations, including the Tiananmen Square crackdown.
Katherine Dong, the daughter of the activist, said that her father was arrested by Vietnamese officials on 24 August in Hanoi.
Ms Dong who is now a Canadian citizen has appealed to the Justin Trudeau government to help free her father and has written to the Chinese and the Vietnamese ambassadors' to ensure his release.
The Independent has learned that while the Canadian government was in contact with their Vietnamese counterpart over Mr Dong's arrest, Vietnam authorities stopped cooperating a month back. The charges of his arrest were still unknown to the family.
The activist was sentenced to prison for the second time in 2014 for more than eight months for participating in a memorial for victims of the Tiananmen Square crackdown. In 2015, Mr Dong escaped to Thailand where he was later joined by his wife and daughter.
The family sought refugee status from the UN’s refugee agency in Thailand and was accepted for resettlement in Canada. However, Thai authorities arrested Mr Dong for "violating immigration laws" and deported him to China.
Beijing imprisoned him for “inciting subversion of state power” and “illegally crossing a national border”.
Following his three-and-a-half-year jail term, Mr Dong fled to Vietnam in 2020, where he had been in hiding, while his family moved to Toronto.
"We are devastated," Ms Dong said. "It has been agonising to be apart from my father for seven years."
The Canadian government has issued Mr Dong travel documents and was negotiating with Hanoi to grant him permission to leave when he was arrested, said Alex Neve, a senior fellow at the Graduate School of Public and International Affairs, University of Ottawa.
His family became alarmed nearly a month back after the Canadian authorities said they had no information about the activist’s whereabouts.
“The family has already been through that trauma once,” he said, adding that they were “hopeful” earlier this year that things were “falling into place” and Mr Dong would be able to join them in Toronto.
Mr Dong’s daughter, in a letter to the Chinese and Vietnamese authorities, has pleaded for his father’s release.
“We are now very afraid that he may have been turned over to Chinese officials. If that has happened, he faces a very serious risk of human rights violations in China,” she wrote in a letter addressed to the Vietnamese envoy.
Ms Dong added: “I have so much admiration for who he is and for his principles. But more than anything, I so very much miss having my father in my life.”
“My mother and I plead with your government to please provide my family with full information about my father.”
A spokesperson from the department of Global Affairs said “Canada is deeply worried” about Mr Dong.
“Officials are working to ascertain his whereabouts, including through diplomatic engagement with both Vietnam and China,” Genevieve Tremblay told Mail and Globe.
China has been routinely arresting veteran human rights activists who refuse to stay quiet, said Ed O'Donovan, special adviser to UN special rapporteur Mary Lawlor.
“Xi Jinping has increased crackdown on dissent putting multiple activists behind bars and Vietnam has a history of detaining and deporting refugees,” he added.