Do you have a source on numbers of vets in gun- or drug-running MCs? Aside from Sons of Anarchy, that's something I've never come across in research.

Something you might be interested to learn is that Vietnam vets were surprisingly quick to acclimate to drug-free civilian life. David Courtwright discusses this in Dark Paradise as one reason the last great heroin use epidemic subsided in the mid-1970s.

Actually, more recently Jeremy Kuzmarov has challenged the idea of widespread drug use in Vietnam. His book, The Myth of the Addicted Army, may include a section on vets once they returned to the United States (I haven't gotten around to reading it, sorry!).

EDIT: I didn't mean to imply I doubt that vets became part of criminal MCs, but that it's just something I've never looked into.