Pham Chi Dung, chairman of IJAVN, and two other journalists - Nguyen Tuong Thuy and Le Huu Minh Tuan - were charged with "spreading anti-state propaganda," according Ho Chi Minh City People's court.
Lawyer Dang Dinh Manh, who defended Dung and Tuan, told dpa that Dung was handed a 15-year jail term, while Thuy and Tuan were each sentenced to 11 years in prison.
All three will face a three-year period of house arrest upon their release.
"The court did not accept any of my arguments," the lawyer said. "The sentences are too hard, especially for Le Huu Minh Tuan, as the court only found six of his articles violating the government’s regulations."
Dung was arrested in November 2019 for sharing information deemed "anti-state"; Tuan was taken into custody last June on the same charges.
Thuy stood in as the acting president of the IJAVN after Dung's arrest but was himself arrested just six months after Dung.
Vietnam’s police said the three "engaged in public activity that seriously violates laws, is dangerous, adversely affects social stability and negatively affects security and order."
Vu Quoc Ngu, director of human rights NGO Defend the Defenders, told dpa the three journalists have been kept incommunicado for months and denied access to legal counsel until just before the trial.
Amnesty International's Deputy Regional Director, Emerlynne Gil, said: "This verdict underscores the Vietnamese authorities' utter contempt for free media. It strikes yet another blow to everyone in Viet Nam's right to access independent information.”
The IJAVN, which was founded in 2014 and advocates for press freedom in Vietnam, has been declared an illegal organization by the Vietnamese state.
Dung, the IJAVN’s founder, previously worked for Vietnam’s Department of Internal Affairs and Security in Ho Chi Minh City.
He quit the role in December 2013, after 20 years, saying: "The Communist Party no longer serves and represents the interests of the majority of the people." - dpa