Jerry Lembcke's book _The Spitting Image: Myth, Memory & the Legacy of Vietnam address above...among number of reasons for myth, he cites importance of Hollywood and quotes from Stallone character on ostensible "coming home" (Lembcke also discusses film of that name with Fonda & Voight) experience: "I see all those maggots at the airport. Protesting me. Spitting. Calling me baby killer."
Lembcke has coupla articles on web that, more or less, summarize the book: "The Spitting Image: Myth, Memory & the Legacy of Vietnam" http://www.rlg.org/annmtg/lembcke99.html
"Media Myth: Vietnam Vets & Spit" http://www.tompaine.com/history/2000/09/07/2.html
Michael Hoover