Human rights groups have expressed alarm over the Communist-ruled country's biggest crackdown on activists in years.
Oai, 36, was arrested in January while on probation after a 2013 sentence for activities aimed at overthrowing the government. He had been released in 2015.
On Monday, the people's court in Nghe An, central Vietnam, also found him guilty of resisting security personnel in performing their duty. He was given another four years on probation as well as the prison sentence.
"Oai said he was innocent and he would appeal," his lawyer, Ha Huy Son, told Reuters.
The court made no immediate comment.
Despite sweeping economic reforms and growing openness to social change, Vietnam's Communist Party retains tight media censorship and does not tolerate criticism.