The ruling by a court in central Vietnam is believed by human rights groups to be the largest subversion case in years, The New York Times reported.
The group of bloggers, writers and political and social activists had been accused of being linked to a United States-based pro-democracy group that Vietnam charged was seeking to overthrow the government.
All of the defendants had been held in detention since their arrests in 2011.
Phil Robertson, deputy director of the Asia division of Human Rights Watch said the convictions were "part of an ongoing deepening crackdown" intended to send a message "that Vietnam means business."
HRW said five of the defendants had written in support of freedom of expression and multiparty democracy.
Some of the defendants participated in demonstrations in support of other dissidents on trial.
The Vietnamese government has placed controls on the Internet and worked to limit demonstrations.