At the beginning of direct American involvement in the Vietnam War, the CIA reported that the Viet Cong had placed a $250 bounty on American soldiers. A couple years later, in 1967, Thai communist insurgents offered a $500 bounty for the killing of American soldiers stationed in Thailand (and supporting the war in South Vietnam, Cambodia, and Laos). I have been unable to find any sources mentioning the fulfillment of a bounty (i.e. a killing of a US service member and a payment to the killer). I can say there was no widespread bounty program, indeed it was the Viet Cong or North Vietnamese soldier's job to be a soldier.

These bounty stories are all over American soldiers' memoirs of the war. Some mention a bounty on any American, some on American doctors, some extra for Marines, or extra for Special Forces, and so on. There certainly were some bounties. And I don't doubt that soldiers heard about the bounties. But as you can see, the bounty talk, and a figure of $500 was around quite a bit.

I can only offer an educated guess. It's more likely that these were rumors used as a type of psychological warfare against American personnel. Conversely, however, it became a source of pride or a badge of honor for some ('hey, the VC pay extra for a dead Marine. That's because we Marines are the toughest'). And an element of hagiography is also at play. Almost any mention of the sniper Carlos Hathcock states the North Vietnamese army had a $30,000 bounty on his head. There is no source for any of these claims, and books published in the 1990s cite a much lower figure, which seems to have grown in direct proportion to Carlos' contemporary renown.

In sum, I am sure your father heard there was a bounty on American soldiers where he was stationed or even his particular unit, but I suspect it was likely a psywar tactic and less an actual bounty.

For clarification, who placed the bounty on him?