Her hearing in the northern port city of Haiphong on Friday was closed to Western media and diplomats, who are ordinarily allowed to watch proceedings on closed-circuit television at the courthouse.
An independent journalist, Nghien was arrested at her home in September 2008 while staging a protest against the Vietnamese government's policies toward China, Vietnam's northern neighbour.
She displayed two banners urging the government to take a tougher stance against China in territorial disputes over the Spratlys and Paracels, two island chains in the South China Sea.
However on Friday, prosecutors withdrew charges related to that case and her conviction was based on a separate incident, her lawyer said.
Instead prosecutors argued that she had defamed officials by writing an article alleging they had stolen money intended for relatives of fishermen killed by Chinese patrols in a 2007 incident.
Nghien was also charged with possessing anti-government documents written by other dissidents and giving interviews to Western media.
In her testimony, she acknowledged she committed those acts but insisted they were not illegal, Hai said.
Her trial followed after last week's sentencing of four Vietnamese pro-democracy activists to terms of between five and 16 years in prison.
Le Cong Dinh, a 41-year-old leading human rights lawyer, and Nguyen Tien Trung, 26, a computer expert and blogger, were said to have undertaken "activities aimed at subverting the people's administration".
The two other defendants, Tran Huynh Duy Thuc, 43, and Le Thang Long, 42, internet entrepreneurs from Ho Chi Minh City and Hanoi, respectively, were convicted of the same charge.
The defendants could have been handed the death penalty.
The group was convicted in a one-day trial in Ho Chi Minh City on charges of collaborating with foreigners in a plot to overthrow the government.
Vietnamese authorities periodically launch campaigns against political dissent and the trial was seen by some as the result of jockeying among political factions in advance of next year's Communist party congress, which is held every five years.