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When I looked this up in one book (A Companion to Vietnam War edited by Marilyn B. Young and Robert Buzzanco, Chapter Twenty-Two: Sanctuary!: A Bridge Between Civilian and GI Protest Against the Vietnam War by Michael S. Foley) I found a line:
Perhaps it has been too easy to accept the stories that portray returning soldiers as  mistreated by civilian opponents of the war; images of long-haired protesters spitting on Vietnam veterans or calling them “baby killers” persist in the American consciousness despite a lack of evidence?
This part has a endnote reference:
The image of antiwar protesters spitting on returning veterans has recently been challenged persuasively in Jerry Lembcke, The Spitting Image: Myth, Memory, and the Legacy of Vietnam (New York New York University Press, 1998). Lembcke, a Vietnam veteran and a sociologist at Holy Cross, argues that not one single instance of an antiwar protester spitting on a veteran has ever been convincingly documented.