There are no substantiated instances of spitting on soldiers. Most flights were civilian charters that landed at military fields, and when they landed at commercial fields on the way over and back, military units were not allowed to leave the airport or even the area in which they were more or less quarantined. My plane (a Pan Am charter) landed at Norton AFB in San Bernardino. I got a ride to Hollywood and rented a car. I was wearing my summer white uniform and had a tan that made me look like I’d been on a beach for a year. Nobody looked twice at me, other than a few young ladies who seemed interested.
Military guys were looked at as being tools of the politicians who were waging a proxy war with Russia/China. Protests were aimed at the government, not at soldiers, although most of us saw that as a fine distinction back then. I remember being angry at the protestors, and puzzled/shocked by the flag burning. I was never confronted by anybody, nor did I ever talk to anybody who had had a confrontation. This is over a 23 year career, so I have a pretty large sampling base. Not that confrontation couldn’t have happened. I’m sure there may have been some heated discussions about the US role in Vietnam.
I can tell you for sure that if anybody had actually spit on me, it would likely have been their last act for quite a long while. I would imagine the same would have been true for most military guys. As to where that meme came from, who the hell knows? It was probably propagated by the same people who spread the lies about Jane Fonda: military vet groups with their own political agendas.
While there wasn’t overt hostility, there was a very noticeable indifference to returning military, and certainly not the adoration that is heaped on our military now. No talk of “heroes”. No welcome home galas. Nobody buying you drinks at the bar. Not even a “thanks for your service” in most cases, not that anybody was really looking for any of that, despite the whining you still hear from some vet groups. Part of that was because of the rotational nature of the people fighting the war. There weren’t shiploads of doughboys coming back to parades after a victorious finish to a world war, just constant flights full of rotating units and individuals returning from a “conflict” that seemingly had no stated purpose and no forseeable end. Then, when it did end, it was an embarrassment for the government and soldiers were the face of that embarrassment. The bright red lacquer of “loser” takes a long time to dull, and it’s just easier to not look at it.