I know, it's a hard thing to shake, but it's an urband legend. It never happened.
How many Vietnam vets do you know? Go ask them if any vets got spit on? They'll say yes. Then ask them if it ever happened to them. The answer will be no. Go ask them if anyone they knew got spit on. They'll hem n haw about it, kinda say yes, but if you ask for specific details like the guys name, they won't be able to give it to you.
Because it's an urban legend.
Don't believe that? Go find a story where a vet recounts being spit on. Either from back then or today. Because if I'd been spit on, I'd remember it. I'd remember when and where. But you won't find any.
What you will find is reports by vets saying that they were treated with kindness and sympathy.