A court in the Mekong Delta province of Kien Giang sentenced a man to four years' imprisonment on Monday for distributing propaganda against the state.
Lu Van Bay, 59, will also be put under house arrest for three years after serving the four-year jail term.
Bay was charged with spreading propaganda against the state under Article 88 of the Penal Code.
According to the indictment, Bay wrote eight articles under various pseudonyms and sent them to an overseas Vietnamese website named "Voice of Freedom" in September 2007.
The writings defamed the Communist Party of Vietnam and called for a multiparty system to replace it.
In January 2008, his act was discovered by local authorities, who summoned him for questioning. Bay then pledged not to write any more articles.
However, he continued writing in early 2010 until his arrest in March this year under the pseudonym Tran Bao Viet.
He wrote 16 articles defaming the Communist Party of Vietnam and the state and calling for a multi-party system, the indictment said.
Bay admitted to his acts and asked for leniency at the Monday trial.