The US military used Agent Orange, named after its orange-striped containers, to defoliate areas believed to be harbouring opposing forces.
It contained dioxin, a chemical linked to a variety of diseases and which Vietnam has blamed for a spate of birth deformities.
Vietnamese victims' groups say the US military sprayed about 80 million litres (21 million gallons) of herbicides, much of it Agent Orange, over southern Vietnam during a 10-year period.
But there has been no internationally-accepted scientific study establishing a link between Agent Orange and Vietnam's disabled and deformed, a US embassy spokeswoman in Hanoi said.
Hoang Van Hue helps his dribbling 31-year-old son sit back in his rusting wheelchair. His daughter, 26, hunches over in her own wheelchair, sticking her fingers in her mouth to make a blowing noise.
"I think that my children were affected with Agent Orange from me," their father said.
The former Vietnamese infantryman said he was exposed to the herbicide dropped by American troops during the war which ended in 1975.
Hue and other Vietnamese who say they are victims of the toxin want the United States to compensate them for its wartime "mistake," and vow to press their claim despite a US Supreme Court decision in March.
The top court declined to hear the victims' appeal after a US district court in 2005 dismissed their lawsuit against manufacturers of the herbicides.
"We are angry but we stay calm and will continue our fight," says Nguyen Trong Nhan, vice president of the Vietnam Association for Victims of Agent Orange/Dioxin, which he says represents about three million victims, and was a plaintiff in the lawsuit.
Hue, 61, said his family's difficult life has its origins in his deployment to southern Tay Ninh province in 1969, when his unit passed through an area where "all the leaves of the trees were not there anymore," and they had to use towels to cover their noses.
He and Ung Thi Tam married the year the war ended, and have two adult children who are normal.
At first, everything also seemed fine with their son Hoang Binh Lap and daughter Hoang Thi Ngoc Ha, their mother told AFP during a visit arranged by the victims' association.
But when the youngsters did not start talking or walking, they knew something was wrong.
Today as adults they are still like babies, said Tam, holding a towel she used to wipe her daughter's face.
"I have to feed them. I have to clean them. They don't know when they pee," Tam said, sitting beside her husband outside their tiny home alongside a single-lane country road on Hanoi's outskirts.
"They are our children so we raise them" despite the difficulties, Hue said softly, with a plain face that makes his emotions difficult to read.
He still looks like a soldier, in sandals, olive slacks and a military-style olive shirt.
With Hue's army pension, and government aid for Agent Orange victims, the family receives about 3.4 million dong (200 dollars) a month -- not much, but considerably more than what they had before, said Tam.
She earns some extra income by grinding rice for villagers in the front room of their house, which contains little else except two beds without mattresses.
Nhan, of the victims' group, describes Agent Orange victims as "the poorest people in society" and said the wife typically bears most of the family's burden.
"I am the strongest person in the family," said Tam, 55, a gentleness shining from her face.
Her husband said "91 percent" of his health is poor, partly from Agent Orange which he thinks damaged his stomach, and from brain trauma caused by a bomb.
Yet, he still has to help care for his disabled children.
"He should have some time to rest but he can't even rest," Tam said, wiping tears from her eyes.
Compensation from the US would allow them to hire somebody to help them take better care of the children, she said.
Despite the lack of a definitive study on the health consequences of Agent Orange, the US has given 46 million dollars to assist Vietnam's disabled over the past 20 years, the embassy spokeswoman said.
Vietnamese and US officials last September said they would further cooperate in responding to environmental and health concerns associated with Agent Orange dioxin.
Both sides have identified at least three dioxin hotspots, including in the central city of Danang, and the US is working with other donors to mitigate damage from those areas, the spokeswoman said. She added that the US budget in March doubled to six million dollars the funding for that mitigation effort.
The Vietnam victims' association has not yet said how much compensation it seeks.
Major manufacturers of Agent Orange in 1984 paid 180 million dollars to US veterans without admitting liability.
In late 2007 the government of New Zealand agreed with its smaller number of veterans on assistance worth 30 million New Zealand dollars (17 million US) for problems including cancer, diabetes and birth defects related to Agent Orange exposure.
Nhan said he knows that filing lawsuits against American companies is "not a picnic".
Despite the Supreme Court's ruling there are a number of legal options such as filing their lawsuit in another country, he added, declining to specify the next step in their fight.
"We will continue it as long as is possible, because it is for justice," said Nhan, 79, who fought in the war against French colonialists.
Tam, 55, said she worries that as she and her husband get older, caring for the children will become even harder.
"I'll try my best, up until I cannot take care of them anymore," she said.