I made some search on google,
9,087,000 military personnel served on active duty during the Vietnam Era (Aug 5, 1964 – May 7, 1975). 2 8,744,000 GIs were on active duty during the war (Aug 5, 1964 – March 28, 1973). 3. 2,709,918 Americans served in Vietnam , this number represents 9.7% of their generation.
US population at these time was 210M.
So we can imagine a society where 1/20 people served duty in war. Is this sufficient to make them adventurers? Not sure, but it makes them a good edge over the commoner template.