What if the US explicitly warned Japan in the days of 23-30 June 1941 that any southward move within Indochina, towards Thailand or the British and Dutch empires, or northward to the Soviet Union would lead instantly to a complete oil embargo and a “grave situation” in the Pacific. However, short of these territorial aggrandizement on Japan's part, America was not interested in curtailing trade any further Furthermore, the warning could add that once an American embargo imposed, simply rolling back the situation to the May 1940 status quo ante n Asia would not be "simple" if "indeed it were possible at all".
Japan might be deterred through mid-1942 at which point they may either try something or become permanently deterred.
Or it may not be deterred at all.
Japanese behavior may vary depending on how much internal discussion and consensus has been reached on the decision to occupy southern Indochina.