Jack Shafer wrote an interesting article in Slate.com about the topic of Vietnam War veterans being spat upon. This article provides an overview of Jerry Lembcke's 1998 book on the topic, in which Lembcke "doesn't prove that nobody ever expectorated on a serviceman--you can't prove a negative, after all--he reduces the claim to an urban myth."
Here are some relevant findings:
". . . the myth of the spitting protester predates the Rambo movies, but how many vets--many of whom didn't get the respect they thought they deserved after serving their country--retrofitted this memory after seeing the movie?"
"Lembcke uncovered a whole lot of spitting from the war years, but the published accounts always put the antiwar protester on the receiving side of a blast from a pro-Vietnam counterprotester."
Why does this false story get perpetuated, then? According to Shafer,
"The myth persists because: 1) Those who didn't go to Vietnam--that being most of us--don't dare contradict the "experience" of those who did; 2) the story helps maintain the perfect sense of shame many of us feel about the way we ignored our Vietvets; 3) the press keeps the story in play by uncritically repeating it, as the Times and U.S. News did; and 4) because any fool with 33 cents and the gumption to repeat the myth in his letter to the editor can keep it in circulation."
On The Media aired a segment on this topic as well, and interviewed Lembcke.
Here is a review of Lembcke's book on the website of Vietnam Veterans Against the War. The reviewer suggests another reason why the myth was perpetuated:
"Perhaps most important in producing the myth were political machinations. The image of the Vietnam vet in the early 1970s was strongly anti-war. There is no place in the American memory for the factually accurate image of vets throwing their medals back at Congress. This image had to be changed if the United States ever wanted to go to war again."
On the opposite side of these conclusions, here's a lengthy rebuttal of Lembcke's claims, based, in part, on Bob Greene's book Homecoming. I'm not sure what to make of this.