Poking around in a sale bin at my neighborhood grocery store, I found a four DVD set of documentaries: Vietnam: America's Conflict (Mill Creek Entertainment, 2009). I suspect that some or all of these are readily available free elsewhere.
The first in the series is Why Vietnam? (1965) put out by the Department of Defense to highlight aggression by the communists in North Vietnam. The 31 minute film begins with a story of the failure of British Prime Minister Neville Chamberlain to halt Adolf Hitler's aggression. Chamberlain failed to heed the lessons of Benito Mussolini's aggression in Ethiopia, the narrator explains.
The Defense propaganda film--documentary is an inaccurate term--is available at Internet Archive's Movie Archive. There, FedFlix "feature[s] the best movies of the United States Government, from training films to history, from our national parks to the U.S. Fire Academy and the Postal Inspectors, all of these fine flix are available for reuse without any restrictions whatsoever." Using "Vietnam" as a search term produces 170 hits. It seems more than likely that I can find most, if not all, of the fifty films on Vietnam: America's Conflict there.