Nguyen Quang A was a founding member of the IDS
Vietnam's only independent think tank has disbanded due to a government decree restricting the right to conduct research on the ruling Communist Party.
The decree came into effect on Tuesday and limits political research to certain approved topics.
The think tank, the Institute of Development Studies (IDS), called the government's actions a blow to intellectual freedom.
The new decree follows recent moves to suppress blogging inside the country.
IDS was an influential body in Vietnam, and frequently questioned the government's views on policy.
But a new government directive - known as Decision 97 - prohibits the institute's researchers from discussing their research openly, and therefore sharply curtails its activities.
According to the decree, which was signed by Prime Minister Nguyen Tan Dung, opinions "opposing the line, objectives and policies of the party and state must be sent to the relevant authorities within the party or state and may not be announced publicly".
"We are not happy at all," IDS president and founding member Nguyen Quang A told reporters.
"We are very sad and it took a lot of consideration to make a very hard decision," he said, referring to the organisation's decision to disband.
The organisation includes 16 of Vietnam's most prominent intellectuals, many of whom have served in the government and are Communist Party members.
"As individuals, all 16 of us will carry on the research work, independently of course, and each will have to find a way to speak up," Nguyen Quang A told the BBC Vietnamese Service.
"Perhaps today, as we are not linked to any organisation and cannot be accused of being part of any organisation, we will make even more critical opinions than before."