In general, they were treated as outcasts and demonized by many whom they encountered back here in "the world."
Specifically, many had difficulty adjusting without those whom they had grown so close to over in Vietnam. Their families expected them to be the same, and they wanted to be the same, but are unable to do so. Norman Bowker is a good example. He writes Tim and tells him that he can't keep a job; he's restless; no one understands him or seems to care about understanding him.
Unfortunately, Tim O'Brien's portrayal of Americans' treatment of Vietnam Veterans is spot-on. They never received a hero's welcome, and for many of them it is too late for us to try to make up for that.