Vo Ban Tam, a Viet Cong veteran, remembers the first time he encountered John Kerry on the banks on the Bay Hap river, a day that ended in bloodshed.

Almost half a century later, the Mekong delta shrimp farmer, now 70, grasped hands with the US secretary of state in mutual respect.

Kerry returned to the Bay Hap river at the end of a visit to Vietnam, less than a week before he leaves office, looking for the spot where he won a Silver Star medal for bravery as a young navy lieutenant.

On 28 February 1969 as the skipper of Swift boat PCF-94, Kerry was on patrol when Vo’s unit launched an ambush.

The plan, Vo told his former adversary on Saturday, was to use rifle and grenade fire to lure the heavily armed US patrol boat within range of a shoulder-held rocket launcher.

This tactic had paid off for the Viet Cong in the past, but Kerry, then 26, made a dramatic decision, deliberately beaching his boat then storming ashore to pursue the enemy.

John Kerry identifies the location of the skirmish for which he was awarded the Silver Star. Photograph: Alex Brandon/AP

Grabbing an M-16 rifle, he chased down the guerrilla with the rocket launcher and shot him dead, saving his crew from a counterattack.

Vo remembered the dead man, 24-year-old Ba Thanh, as a respected member of the Viet Cong’s main force in Ca Mau province, specially trained to use the prized launcher.

“He was a good soldier,” he recalled, speaking through an interpreter on the banks of the river, shortly after Kerry had revisited the scene of the ambush for the first time.

Kerry had not known the name of the man he shot. During his unsuccessful 2004 campaign for the White House, opponents tried to tarnish his war record by claiming he had killed a teenager.

US officials preparing for Kerry’s visit tracked Vo down, and his account confirmed the US secretary of state’s memory that his adversary had been an adult.

Vo acknowledged that Kerry’s actions had thwarted the Viet Cong that day, but he also recalled proudly how his comrades had often had the upper hand.

“We were guerrillas. We were never there where you were shooting,” he said, telling Kerry they had heard his boat coming from nearly a mile away.

John Kerry testifies before the Senate foreign affairs committee in 1971 as director of a veterans’ campaign against the Vietnam war. Photograph: CSU/Everett/Rex

“Well, I’m glad we’re both alive,” the secretary of state said.

Kerry returned from Vietnam later in 1969, and the holder of Silver and Bronze Stars for bravery and three Purple Hearts for being wounded in action went on to become a prominent anti-war activist.

The Yale graduate stood out among his fellow veterans and his devastating testimony before a Senate committee in 1971 sealed his celebrity. He forecast that Washington’s search-and-destroy missions and other brutal measures would fail to overcome the Vietnamese determination to resist foreign occupation.

On behalf of his fellow soldiers, he famously demanded: “How can you ask a man to be the last man to die for a mistake?”

Kerry went on to become a senator, presidential candidate and finally secretary of state, helping to lead post-war reconciliation efforts.

As Donald Trump prepares to assume the US presidency, bringing Kerry’s tenure at the State Department to an end, he plans more visits to Vietnam.

He wants to work on environmental problems with the Lower Mekong Initiative, and is involved with fellow veterans in plans to open a US-funded Fulbright University in the country.

Asked how it felt to be back at the scene of his day of battlefield success within a much greater American failure, he replied: “It’s weird. It’s a little surreal.”