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.
Foreign media were barred from the courtroom, although they were allowed to watch proceedings via a video link.
The group was arrested in June last year along with Tran Anh Kim, a former army officer who was sentenced last month to five and a half years in prison for subversion.
The four were accused of collaborating online with foreigners to promote democracy and joining the banned Democratic Party of Vietnam.
Dinh, who studied law in the US on a Fulbright scholarship, was accused of going to Bangkok to attend a seminar on nonviolent political change organised by Viet Tan, an international pro-democracy network with members inside Vietnam.
According to Vietnamese authorities, police also found a draft of a new Vietnamese constitution when they raided Dinh's Ho Chi Minh City law office in June.
Vietnam's government has labelled Viet Tan a terrorist organisation.
The latest case is the most high profile in a year-long crackdown on dissidents and bloggers.
Vietnamese media had quoted prosecutors as saying the charges against Dinh constituted a "particularly serious violation of national security".
Carlyle Thayer, director of the Australian Defence Academy in Canberra, told Al Jazeera that the government had been determined to make an example of the four in a show trial.
|Trung, right, met George Bush, the former US president, in 2006 [AFP]
"These individuals have taken protests in Vietnam a step further than it has ever been," he said.
"They designed a political party, they have mapped out a strategy and tactics and have particularly targeted dissidents within the communist party and tried to win them over in an attempt to challenge the party by non-violent means ... Now that this movement has emerged, they are determined to crush it."
On the eve of the trial, Viet Tan issued a statement condemning what it called the "arbitrary charges" against Dinh and other democracy activists.
"The people of Vietnam have the right to discuss and advocate for the political system that best serves them," it said.
"Viet Tan is committed to empowering Vietnamese to effect social change through peaceful, nonviolent means."
Vietnamese authorities periodically launch campaigns against political dissent and the latest trial is being 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.