Historians and folklorists alike have studied the "military personnel getting spit on" question for close to 30 years now, and guess what? Prior to that first Rambo movie there were no reports. Following the movie? By the end of the '80s it had become common knowledge that members of the military were spit on when they returned from Vietnam. The researchers who have studied this phenomenon have never been able to find a single verifiable incident -- it's always "it happened to a friend of a friend." You know what you call something that's attributed to a friend of a friend? If you're kind, it's an urban legend. Or, if you're me, you just call it bullshit.
The author of the book shown, incidentally, interviewed hundreds of people who initially claimed to have personal knowledge of spitting happening but in the end there was no corroborating evidence (e.g., news reports in print or on television) and it turned out the person making the claim was actually repeating "a friend of a friend" story.
Actually, some of the stories that get cited as "evidence" by the people who swear the spitting happened are so totally bizarre that it's amazing anyone believes them. One story, for example, claims that when service members arrived in Los Angeles, they'd duck into restrooms to change into civilian clothes to avoid being abused by the public. So many uniforms were removed and discarded that trash cans were overflowing with jettisoned Class A uniforms. WTF? If these service members were still in the military and were en route to a new duty station, they were going to need those uniforms. If they'd already been discharged and were heading home, they would have been in civilian clothes. If they were still in the military, sooner or later they'd need their Class A uniform, the one with the most expensive pieces, for some occasion. On the pitiful pay personnel got back then, no one still in the military was ever going to throw a uniform away because they'd have to pay real money to replace the various pieces. Classic sign of an urban legend: it contradicts common sense.
(Side note/minor digression: the uniform story reminds me a lot of the anti-Jane Fonda story about her betraying POWs by getting them to tell her their names and service numbers and then passing that info on to the North Vietnamese. Whatever moron thought that one up apparently forgot that when the POWs were captured they (a) were wearing dog tags with name and serial number, and (b) the one thing every person in the military is told is that when they're captured the only thing they're supposed to tell the enemy is their name, rank, and serial number. Fonda did ask guys their names but it was so she could let their families know she'd seen them and they were okay. She also carried letters from POWs back to the States to mail for them.)
Anyway, I can understand Vietnam era veterans feeling neglected and mistreated, but it was not by the general public. The people they should be pissed at are the paper pushers in the Veterans Administration who refused for decades to acknowledge the harmful effects of Agent Orange, who dithered about recognizing and treating PTSD, and who put up roadblocks to almost every disability claim. Nothing new about that, of course. Veterans have been getting screwed over by an ungrateful government since the country was founded.
The same person who's channeling John Rambo also made the claim that during the 1970's service members were told not to wear their uniforms while traveling because of the low opinion the public had of the military. Again, bullshit. For a brief time members were indeed told not to wear their uniform while traveling on civilian aircraft but it was not because of any anti-military sentiment on the part of the general public. The '70s witnessed record numbers of hijackings; service members were advised not to travel in uniform so they'd blend in with the other passengers. Arriving in Cuba in a U.S. Army uniform would not have been cool. However, as a general rule, including during most of the Vietnam conflict, if the military paid for the plane ticket, you had to be in uniform. The government was (and still is) notoriously cheap so when you flew on a civilian plane, you flew stand-by. If you weren't in uniform, the gate agent stuck you at the bottom of the stand-by list. In uniform you were at the top.
I do feel obligated (as usual) to note that way too many of the people who tend to do the super patriotic thank you for service garbage are the same ones who never had the time or desire to serve themselves and who freak out at the suggestion that maybe their high school age kids think about enlisting. I'm still wishing I could have somehow preserved the horrified look on a supervisor's face in Omaha when I responded to his lament about his no-clear-goals adolescent by saying, "Well, what about the military?" You'd have thought I'd said, "Hey, Don, how about if your kid tries prostitution for awhile?" So please don't thank me for my service (I'm Vietnam era) and I promise I won't call you out as an elitist classist ass who thinks getting shot at by the Taliban is a chore reserved for the low-income kids from the sketchy neighborhoods.