They accused the two lawyers of collaborating with overseas pro-democracy advocates, using the internet to advance their views and of working with Thadeus Nguyen Van Ly, a Catholic priest and perceived dissident, who was sentenced to eight years in prison in March.
Friday's trial passed without major incident unlike Ly's in the city of Hue. The priest was removed from court several times after shouting in protest.
Dai was sentenced to five years and Nhan to four years for advocating a multiparty state in Vietnam.
They must serve four years and three years of probation respectively after their release.
Shortly before the close of the four-hour proceeding Nhan said he rejected the trial saying it was "not right".
Reporters and diplomats were not allowed in the courtroom, but viewed the trial on closed-circuit television.
Ralph Falzone, from the US embassy in Hanoi, said: "We are aware of nothing to indicate that these individuals were engaged in activities that conflict with rights regarding the peaceful expression of political thought widely recognised under international law."
Alexis Andres, a European Union diplomat, said the evidence against Dai and Nhan was "very weak" and the sentences excessive.
"Nobody should be condemned for peacefully expressing their views," he said.
Before they were arrested on March 6, Dai and Nhan provided human rights training to Vietnamese students.
Dai also represented ethnic minorities who were members of Protestant faiths.
On Wednesday, Vietnam released Phan Van Ban, who was jailed for 22 years after he joined an organisation calling for political change.