I had a professor in college who would always say this as fact, which made me believe it. I'm now a history teacher, and have said this to my students in the past, but I have not been able to find any viable sources to support this claim. It's always fun to talk about how the world would have been different if the French were never let back in Vietnam, but I'm not quite sure if that's really what F.D.R. said. Thanks for any help.
On August 14 1941, Franklin Roosevelt together with Winston Churchill released a joint declaration which was to go under the name of the Atlantic Charter. For this question, the sixth clause is the most important one: "After the final destruction of Nazi tyranny they hope to see established a peace which will afford to all nations the means of dwelling in safety within their own boundaries, and which will afford assurance that all the men in all the lands may live out their lives in freedom from fear and want.”
This charter, alongside its sixth clause, had a tremendous effect in the world and while Winston Churchill was quick to reassure his government that the charter did not apply to the British Empire, Roosevelt had meant for it to apply universally. FDR had entered the 1940s with a strong anti-colonialist conviction. This of course extended to the belief that France should give up French Indochina (modern day Vietnam, Cambodia and Laos). FDR felt very strongly about the possibility that Indochina had been used by the Japanese as a springboard for the initial conquests in 1942 (it wasn't) and adding onto his generally bleak opinion about the French after its defeat in 1940, made him very determined to speak out against French colonialism in Indochina.
This does not mean that Roosevelt saw Indochina was being prepared for independence. Roosevelt had at occasions displayed, in the words of Fredrik Logevall, a "patronizing and cryptoracist view of the Vietnamese" and considered them unfit to rule themselves. Instead, he suggested a more paternalistic suggestion in which a trusteeship would be established along the lines of the mandate system of the League of Nations. These former colonies would be helped to independence through a series of stages culminating in total independence.
Roosevelt was very reluctant in allow Free French regular forces to return to Indochina and while he began to look the other way as Free French forces were deployed with the help from the British towards the end of his own life (and the war), Roosevelt still remained convinced that allowing France to return to Indochina was a bad idea - even though it began to falter due to the realities of the war itself. To Roosevelt, the final (and inevitable) goal was universal decolonization.