I know there were plenty of private contractors in Vietnam, but I'm specifically curious about security forces and the functions they served, particularly those that would be akin to the contemporary Blackwater (or Xe, or Academi, or whatever they call themselves now).
There are no indications at all that PMC's were used during the Vietnam War. The modern PMC's trace their lineage to the 1960's when several new companies offering private security services were formed and used in conflicts in the Middle East as well as in Africa. Unfortunately, a discussion as to why the rise of PMC's happened in the last 20 years is outside of the scope of this subreddit.