You hear all these stories about what a bad ass he was in Vietnam etc .. the "blood and guts" routine he and Troutmsn refer to. Well. Let's see it.. not that we haven't in the past films but I just think it'd be a good idea. Naturally Stallone or Crenna can't be in the movies but the possibilities are there... I don't think Vietnam War movies are in vogue at the moment though.
There is a prequel. It's called "The Vietnam War." But to be serious, we don't need a proper prequel because everyone knows how badly that war screwed up its soldiers. If you know anything about it, Rambo's behavior in the first film is no mystery.