You're misunderstanding those numbers which compare Vietnam era vets against the vets of other war eras and not against the general population.<quoted text>"Vi
etnam War-era veterans were
the most common wartime veterans in both State (36%) and
Federal (39%) prison. Veterans of the Iraq-Afghanistan eras
comprised 4% of veterans in both State and Federal prison.
The average length of
military service of veterans in prison
was about 4 years. An estimated 62% of veterans received
an honorable discharge and 38% received various types of
other discharges." (Bureau of Justice, 2004)
It's basically an exercise in fun with numbers as the pre Vietnam legal system was much kinder and gentler and drugs weren't yet perceived as a problem. A lot of the stuff folks used to do for fun back in the "good old days" is a felony "now a days."
About 2.6 million Americans actually served in Vietnam, there were many many more that served during the so called Vietnam era.