Analysts say the move is inevitable given Washington's failure to atone for its use of the toxin.
The Vietnam Association for Victims of Agent Orange, which was established last month under the umbrella of the ruling Communist Party, filed a legal case on their behalf at the US Federal Court in Brooklyn, New York on 30 January.
Nguyen Trong Nhan, the organisation's vice president, said more than 20 American companies engaged in the production of Agent Orange had been named in the case, including Dow Chemical Co and Monsanto Co.
The amount of money the three plaintiffs are seeking in damages has not yet been determined, Nhan said.
"This has been a long time coming. It was almost inevitable because the US government has done almost nothing to help Vietnamese victims," said Chuck Searcy, an American aid worker in Vietnam.
Nhan insisted the legal case was initiated independently from the government. Some sources say it could have been filed at the behest of American lawyers working on a pro bono basis.
"It looks as though the government is watching from the sidelines without intervening to block the case. It could probably do so if it wanted to," said Searcy, a Vietnam War veteran.
His comments were echoed by a Hanoi-based Western diplomat.
"I don't think the government is behind this. They seem sincere in their desire to continue the process of improving US-Vietnam relations," he said. "I'm just surprised it hasn't happened before."
Source of contention
The legacy of Agent Orange remains a source of contention between the two former foes, who only established diplomatic relations in 1995, two decades after the war ended.
From 1961 to 1971, the US and South Vietnamese military sprayed millions of litres of toxic defoliants, mainly Agent Orange, over South Vietnam to destroy the vegetation used by communist forces for cover and food.
Exposure to the chemical causes
deformities to develop in the womb
Various herbicide mixtures identified by coloured stripes on their containers, were used during the spraying programme, which was known as Operation Ranch Hand. Agent Orange was the most common mixture used.
Hanoi says the defoliant has caused health problems for more than one million Vietnamese and continues to have devastating consequences.
A study, released in August last year by scientists from the United States, Germany and Vietnam, found that Agent Orange was still contaminating people through their food.
Dioxin, the defoliant's deadly component, can cause an increased risk of cancers, immunodeficiencies, reproductive and developmental changes, nervous system problems and other health effects, according to medical experts.
Dioxin, the defoliant's deady
component causes health defects
Vietnam says the United States has a moral and humanitarian responsibility to heal the wounds of the war, but it has never formally asked for compensation for Agent Orange victims.
Washington, however, insists there is no direct evidence linking dioxin with any illnesses. Agreeing to disagree, both governments signed a pact in March 2002 on a framework for more research into the impact of the defoliant.
"Despite the US government saying there is no scientific evidence proving the impact of Agent Orange on human health here, the evidence seems pretty overwhelming," said Searcy.
US chemical companies engaged in the production of Agent Orange, including Dow Chemical and Monsanto, have found themselves in the dock before.
In 1984, in a class action settlement with no admission of liability, manufacturers agreed to pay 180 million dollars to US war veterans who died or became ill after exposure to Agent Orange or other defoliants.
But for years US veterans have been seeking additional compensation to that settlement, and in June 2003 the Supreme Court ruled they could continue to pursue claims against the manufacturers despite the earlier settlement.
"This has been a long time coming. It was almost inevitable because the US government has done almost nothing to help Vietnamese victims"
American aid worker in Vietnam
"There is a delicious irony about this lawsuit," said Carl Thayer, a Vietnam expert at the Australian Defence Force Academy.
"In the wake of the catfish and other bilateral disputes, Vietnam finally appears to be learning to play the legal game like the US does."
After a year-long legal tussle, Washington slapped punitive tariffs on imports of Vietnamese frozen catfish fillets in July last year, saying they were hurting the US catfish industry.