Yeah, Japan's sole interest was cutting off Chinese supplies it's the only reason they went into Indochina in the first place, granted I would still that going for Indochina would be worrying for the U.S, or at very least Roosevelt.
Without the China War, Japan has zero interest in occupying Indochina. It's not especially valuable in its own right, and it's fairly isolated from Japanese holdings.
OTL they occupied it as a base to cut the supplies coming to Chiang, here that's not an issue.
I think "zero interest" is an overstatement. The territory has value as a producer of rice and rubber, and with it being "orphaned" under weak Vichy control, its an even weaker target than China. While Japan won't be able to deploy there from Hainan, as in OTL, Taiwan is not super far, Japan has carrier air power and regularly patrolled around (and claimed) the Spratlys. Heck, I think a general Sino-Japanese war could still be avoided even *if* Japan grabbed Hainan island at some point. The Chinese anticipated a Japanese occupation at many points between 1932 and when it was actually occupied in early '39, and all Chinese knew China didn't have a fighting chance in a *naval* batle against Japan.
If somehow the war with China is avoided (which requires a drastically different Japanese military and political situation) and the Japanese do somehow end up occupying Indochina, then the next move is likely...China.
Well the situation requires the Japanese to be less further along the crazy spectrum than OTL but it doesn't need to remove all Japanese adventurism, radicalism, militarism and opportunism.
Your point about the next move being against...China, is a very plausible one however. A Japan that has avoided war with Japan through 37, 38 and most of 39 may find that it sees the outbreak of war in Europe in late 39 or at least the fall of France and Battle of Britain as an opportune moment to settle scores with China. If they have not started to do so before the fall of 1940 and occupy Indochina first, they might see it as providing another angle of attack into China.
A Japan that just starts full-scale war with China in 39 or 40 will likely be much less likely to get involved any further in the global war.
Of course, having gotten this far not having gone all-in against China, the global war and the Asian territories of the various European states engaged in war against Germany (UK, Netherlands, after June 41, the USSR, and the US as a quasi-belligerent), might become the "bright, shiny objects" to grab the attention of Japanese NAvy *and* Army leaders and planners.
US policy towards Japan was always more focused on protecting China (and later the Allies) than it was about Indochina;
I agree that Indochina itself, considered in a vacuum, didn't rate much interest in the US. However, it is notable that as long as there was just Sino-Japanese fighting the US went no further than a half-assed "moral embargo" on high-grade aviation gas and high-grade steel. When Japan occupied northern Indochina however, the embargo was extended to all scrap iron. When Japan occupied the remainder of Indochina it was an all-out fuel embargo and asset freeze.
if it somehow became Japanese occupied outside of the greater context of the China War, the US probably wouldn't especially care.
I think the context of the European-global anti-Hitlerite war was more determinative than the the context of the China. In the ATL while there will be less accumulated anger with Japan, once Japan occupies Saigon and Da Nang, America will be disturbed by the potentially threatening implications of that forward Japanese position for the security of the British Empire in Asia. southern Indochina is a useful forward base against Malaya, Singapore, the DEI and Philippines.