Most of the abuse leveled at veterans returning from Vietnam was done by groups like the American Legion and was directed at the Winter Soldiers, VVAW and those who threw their medals back.
The most recent incidence of spitting on Vietnam War Veterans occurred in 2004 when the Republican "Swift Boat" campaign figuratively speaking spit on decorated Vietnam War Vet, John Kerry.
From Common Dreams: http://www.commondreams.org/view/2012/06/01-7
Out of all the status-quo-sustaining fables we create out of military history, none are as enduring as Vietnam War myths. Desperate to cobble a pro-war cautionary tale out of a blood-soaked tragedy, we keep reimagining the loss in Southeast Asia not as a policy failure but as the product of an America that dishonored returning troops.
Incessantly echoed by Hollywood and Washington since the concurrent successes of the Rambo and Reagan franchises, this legend was the central theme of President Obama's Memorial Day speech kicking off the government's commemoration of the Vietnam conflict.
"You were often blamed for a war you didn't start, when you should have been commended for serving your country with valor," he told veterans. "You came home and sometimes were denigrated, when you should have been celebrated. It was a national shame, a disgrace that should have never happened."
It's undeniable that chronic underfunding of the Veterans Administration unduly harmed Vietnam-era soldiers. However, that lamentable failure was not what Obama was referring to. As the president who escalated the Vietnam-esque war in Afghanistan, he was making a larger argument. Deliberately parroting Rambo's claim about "a quiet war against all the soldiers returning," he was asserting that America as a whole spat on soldiers when they came home — even though there's no proof that this happened on any mass scale.
In his exhaustive book entitled "The Spitting Image: Myth, Memory, and the Legacy of Vietnam," Vietnam vet and Holy Cross professor Jerry Lembcke documents veterans who claim they were spat on by antiwar protestors, but he found no physical evidence (photographs, news reports, etc.) that these transgressions actually occurred. His findings are supported by surveys of his fellow Vietnam veterans as they came home.