It has triggered criticism from the US embassy.
Huynh Trong Hieu said on Wednesday that police detained him at Ho Chi Minh City airport on Sunday night as he was checking in.
They questioned him for two hours and took his passport, which had a valid American visa. He was then released.
"I was prepared for the fact that they would ban me from leaving the country as they had prevented many people who dared to promote democracy and human rights in Vietnam," Hieu said by telephone. "By banning those people from leaving the country, the government wants to give a warning to others that they have the authority to decide the fate of all its citizens."
Vietnamese authorities declined immediate comment.
Vietnam converted to a market economy in the late 1980s and wants to integrate with the world, but maintains strict controls on freedom of speech and political expression. Bloggers, activists and others are routinely arrested and imprisoned.
Foreign media representatives are allowed to live in the country but are subject to restrictions on where they can travel and what they can report.
Hieu was flying to the United States to receive a Hellman-Hammett award from Human Rights Watch on behalf of his father Huynh Ngoc Tuan and sister Huynh Thuc Vy, who are both prominent bloggers.
Each year, the group gives cash grants to selected writers for their commitment to free expression in the face of government persecution.
Hieu said he was traveling on behalf of his father and sister because they both knew they wouldn't be allowed to leave.
The United States is seeking closer ties with Vietnam, in large part because it shares concerns with Hanoi over China's increasing assertiveness in Southeast Asia.
But it is vocal in its criticism of the country's human rights record, which most observers say is getting worse.
"We are troubled by the intervention of Vietnamese authorities to prevent Huynh Trong Hieu from traveling to the United States," the embassy said in a statement.
"We urge the Vietnamese government to lift travel restrictions on Mr. Hieu and take steps to allow his family and all Vietnamese to peacefully express their views without fear of retribution." – Sapa-AP