Where did you hear they did?

Hi there -- you may be interested in this older thread on the topic, particularly the answer from u/Bernardito.

Generally speaking, the idea of Vietnam veterans being spit on is considered to be a myth. That's not to say that it never happened (that's next to impossible to prove), but most of the stories are undocumented, didn't lead to arrests or specific newspaper articles about the incident, happened to "a guy i knew's cousin", etc. While there was a lot of anger about Vietnam toward the end of the war, much of that was directed toward government officials (over things like the Tet Offensive and the bombings in Cambodia and Laos) rather than the soldiers themselves.

Jerry Lembcke argues in The Spitting Image that these stories of protesters spitting on returning veterans were fueled by the Nixon administration as a way to de-legitimize the anti-war Left/youth movements, and to prevent a political alliance between anti-war protesters and the veterans themselves, many of whom were also disillusioned with the war and with the government. In general, Vietnam led to a lot of ideological battles over who was "to blame", whether it was actually a loss, etc., and the "spitting on soldiers" myth appears to be a major piece of that puzzle.

Sources:

Jerry Lembcke, The Spitting Image: Myth, Memory and the Legacy of Vietnam

Also, see: Michael Allen, Until the Last Man Comes Home: POWs, MIAs and the Unending Vietnam War for more about Vietnam memory