Last year, Dick Cavett’s Watergate dove into the veteran talk-show host’s program archives to explain and explore that Seventies flash point from a surprisingly rich and varied selection of perspectives. Tonight, he does the same thing with another incendiary subject of the era: the Vietnam War. And once again, he proves that, many decades ago, he presided over a talk show that on occasion, talked about subjects of utmost seriousness.  Check local listings.