I remember first coming across this idea that the US had supported the Vietminh from a scene in Apocalypse now. And then vaguely recall trying to look up whether there was any truth to it and I think (correct me if I'm wrong) that Bernard Fall asserted that there was a period of US support for the Vietminh against French colonialism but without much in the way of support.
Am I remembering this correctly? What was the Vietminh/US relationship like in the early years after World War II? Have any historians looked into this in greater depth since Fall?
The United States, through the OSS (the Office of Strategic Services, a sort of proto-CIA) had armed and trained various Vietnamese rebel groups to fight the Japanese occupation troops in Vietnam. Japanese policies (i.e. forcible exports of rice) in Indochina had lead to the starvation of about a million people in Indochina. Plus, many Vietnamese were becoming staunch nationalists and anti-colonialists and wanted the Japanese out.
Sensing a chance to harras the Japanese, the US sent OSS trainers and weapons to help Vietnamese resistance groups, some of which later ended up as the Viet Minh.
After WWII, with the Cold War in full swing, the US becomes worried about the communist Viet Minh taking control of French-held Indochina. As a result, the US heavily supports the French. At one point, the US even mooted sending bombers to help French troops trapped at Dien Bien Phu.
I think you may be confusing US foreign policy during the Cold War as US support for Vietnam during French colonization. During the Cold War both the US and USSR had strong anti-colonization policies which would've meant US and French disagreements over Vietnam. This foreign policy can best be shown in the strong US/USSR backlash during the U.K./French invasion and brief retaking of the Suez Canal.
The US didn't have much, if any, relations with Vietnam right after WW2. It wasn't until communism spread to Vietnam that the US took notice due to its policy of containment.