Luu Van Vinh, 52, identified as the group leader, had been sentenced to 15 years in jail at the trial of first instance last October, and four accomplices were given jail terms of eight to 13 years each.
They appealed their sentences.
The Monday court, however, rejected their appeal and upheld the original ruling, saying that the defendants' actions were ‘extremely serious’, violating national security and attempting to undermine the existence and strength of the people’s administration and the State of Vietnam.
These actions merited strict punishment, the court held.
After serving their prison sentences, the defendants would be placed under house arrest for another three years, the court ruled.
According to the indictment, Vinh and his accomplices worked together to establish an organization called the "National Alliance of Vietnam."
The organization criticized and distorted the policies of Vietnam’s Communist Party in an attempt to overthrow the administration.
The defendants were arrested in November 2016 after police caught them distributing documents promoting the organization at a church in Ho Chi Minh City’s Tan Binh District.
Investigators subsequently identified Vinh as the group’s mastermind, responsible for setting goals and policies for the "National Alliance of Vietnam" as well as recruiting members for the illegal organization.
Other defendants, meanwhile, were found guilty of serving as his advisors in expanding and financing the organization, encouraging and strengthening its new members' belief and making preparations for the organization to establish its own army.
Under Vietnam's Penal Code, "carrying out activities aimed at overthrowing the people's administration" is a crime punishable by 12-20 years in prison, life imprisonment or the death sentence for organizers, instigators and active participants, while accomplices could face 5-15 years in prison.
Last year, the Ho Chi Minh City People’s Court sentenced Nguyen James Han, a 49-year-old man with dual American-Vietnamese citizenship, and Phan Angel, a 62-year-old American national of Vietnamese origin, to 14 years in prison followed by immediate deportation for attempting to "overthrow the people’s administration."