This may have been the difference between Vietnam and other wars in former colonies. Where the British or French turned over authority to locals, most of those governments could crush communist rebels. In Vietnam, they already had half the country recognized as their own and undercut the security and legitimacy of any goverment we could get.
We could defeat them in the field, but could we stay in Vietnam for years and dercades to keep them from coming back? The good news is that the seventies were the high water mark and the beginning of the end for communism.
Also in Vietnam you had the Vichy French forces in Vietnam cooperating with the Japanese. Then when the French switched sides the Japanese tossed them over. That made a big impression on the Vietnamese. On top of that, it was the Viet Minh who took over Vietnamese institutions and cities when the Japanese surrendered in 1945. Then the French came back and asked for it all back. The French did this in part to restore their faded glory (which was lost in 1940). Oddly enough, even the French Communist party at that time demanded that the French return to Vietnam. Go figure.