Presiding Judge Bui Xe read out the sentences late Friday morning, after a trial in the city of Hue that lasted just four hours.
The judge sentences Catholic priest Nguyen Van Ly to eight years in prison. Two of Ly's co-defendants, Progression Party founders Nguyen Phong and Nguyen Binh Thanh, receive six and five years.
Two female associates, Le Thi Le Hang and Hong Thi Anh Dao, received suspended sentences.
The defendants were found guilty of spreading propaganda against the Vietnamese state. For the past year, they have been publishing an independent newsletter called "Freedom of Speech." Last September, they established the Vietnam Progression Party as an alternative, non-communist party.
Ly testified only briefly at the trial, and his responses to questions from the judges were twice cut short when he tried to make proclamations.
"Vietnam practices the law of the jungle!" Ly shouted.
The police cut his microphone and hustled him out of the courtroom.
The defendants had no lawyers, and were not given an opportunity to present a defense. At the end of the trial, they were asked whether they would like to say anything in response to the state's case. Nguyen Binh Thanh began to make his case.
Thanh says his actions were in accordance with international law. But the presiding judge cuts him off, too, saying that is not relevant.
A Western diplomat attending the trial said the results appeared to be a foregone conclusion. Another diplomat, from the United States, read out a statement afterwards.
"We call upon the Vietnamese government to allow individuals to peacefully exercise their legitimate right to freedom of speech without fear of recrimination," said the diplomat.
Despite the cursory nature of the proceedings, this marked the first time diplomats and foreign journalists have been allowed to attend the trial of a dissident in Vietnam - an unusual move towards greater openness in a still-tightly controlled society.