Dinh Nhat Uy, 30, was convicted at the end of a one-day trial in the southern province of Long An on charges related to an Internet campaign against his brother's imprisonment for spreading anti-government propaganda, his lawyer said.
Uy's case was apparently the first time a Vietnamese activist has gone on trial only for comments made on social media.
In Vietnam, convicts serving suspended sentences are effectively placed under house arrest, with severe restrictions on their movements and a requirement to check in regularly with police.
"I told the court that Uy was innocent, that the charges against him were not objective," he said, adding that he had called for Uy's immediate release.
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"He is the victim of an injustice," he added.
Uy was sentenced for violations of article 258 of the penal code, which covers "abusing democratic freedoms against the interests of the state".
The charges, regularly used by authoritarian Vietnam to silence dissidents and activists, carry a maximum sentence of seven years in prison.
Vietnam, branded an "enemy of the Internet" by Reporters Without Borders, is a one-party state that bans private media and controls all newspapers and television channels.
According to Uy's indictment -- a copy of which was posted online on the banned but popular blog Dan Lam Bao -- he was charged solely for Facebook postings. Usually charges for dissidents and activists relate to blog postings.