Cultural concern for veterans coming home and being treated poorly, or being unable to readjust to civilian life, definitely predates the Vietnam war.
Consider "The Best Years of Our Lives"[0] from 1946, set after WW2.
https://utpress.utexas.edu/books/rivas-rodriguez-texas-mexic...
http://www.huffingtonpost.com/2013/11/11/felix-longoria-affa...
The Felix Longoria affair in which a Mexican-Texan veteran was denied who died in the war was denied burial by the local funeral. The American G.I. Forum an association of mexican american vets helped organize a protest and sent a letter asking help from Senator (and future president) LBJ, who arranged to have him buried in Arlington and initially publically supported the veteran but later ducked and covered from the conservative backlash. The G.I. Forum would become a powerful civil rights organization over the years.
https://en.wikipedia.org/wiki/Isaac_Woodard
Isaac was an African American World War II veteran who was beaten while still in uniform (hours after an honorable discharge) until he was blinded (because he had talked back to the bus driver when the bus driver refused to let him use the bathroom. the driver called the cops on him). This was a major scandal that helped inform the nation and especially President Truman about the evils of Jim Crow but he couldn't pass civil rights laws past the senate. All he was able to do was desegregate the army and federal goverment by executive order.
"Johnny I hardly knew ye" was first published in 1867 and echoes similar concerns.