Jodel brings up a interesting point.
Japan could play the "let me take care of your colonies" game like USA did with Iceland. No war with China, yet chaos and civil war in China so Japan moves in to stabilize a derelict Indochina (perhaps even use the Indochina war with Thailand as a further excuse). What could the allies say? Especially when the Vichy invite the Japanese in?
It was no secret USA was not favorable to the European regimes ruling over Indonesia, etc. Rebellion fostered by the Japanese and then them becoming those new countries patrons would give the Japanese their co prosperity sphere without war.
With Indonesian oil and Korean/Manchurian coal and steel, the Japanese Empire continues its industrialization and if the allies tolerated Facist Franco in Spain in the cold war era, how much more would they value a Facist Empire of Japan watching over Communist East Asia? Who better to keep those dominoes from falling in Korea, Indochina, etc...???
Asia for the Asians, and capitalist/right-wing Asians >>>> Viet Cong and whatever the other movements were called in the rest of Asia.
The Empire of Japan could of been used as a anti-communist model and sponsor for the rest of Asia. Instead we got various western puppets like Sygnman Rhee and Nyo Dien Zim.
Indeed, and thank heavens for that; both of the examples of puppets you give (though I assume you mean Ngô Đình Diệm by the latter; "Zim" isn't even a Vietnamese name) were weak enough that, in spite of their authoritarian brutality, they were both eventually and relatively painlessly toppled and replaced in the early 1960s, regardless of "East" (the ultimate victors in Vietnam) or "West" (likewise in Korea). Just musing in a bit of stream-of-consciousness, but a powerful fascist Japan would have only continued to maintained these territories as resource-bases for their industry and population, for all that they established those factories abroad as well as in the Home Isles; this would have caused them to stagnate economically, and unlike the strongmen you list, Japan might have been strong enough to continue to enforce this status due to an untouchable, distant, and continuously loyal base of power in the near term (say, a decade or two). Even if Communist-funded insurgencies supplied by the USSR and PRC across the shared land borders with the latter began and grew powerful as in the case of those against several European colonies in Africa, the Japanese perception of Korea in particular as an inviolable "lebensraum" critical to fuel their economy, to borrow the term, would only strengthen their resolve, and if it succeeded in causing significant losses, would only force ever more powerful encroachment by the military in Japanese civilian life until eventually something gives, much as in Portugal with the Carnation Revolution or in Spain after the death of Francisco Franco. However, unlike Portugal, it would not be the military supporting a democratic coup, but rather the exact opposite, and unlike Spain, there would be no single powerful figure in the military whose death would signify the collapse of the system; it would likely be a bloody, bloody affair not only on the Home Isles, but also and especially in the colonies where decades of resentment would crystallize in an instant. Even historically, South Korea before Park Chung-hee was in many respects as poor as or poorer than Africa; a Korea that wins its independence late, only after a long and bloody conflict sore with massive inflicted wounds and trauma, under a united Communist government such as the one that so tremendously mismanaged their originally-superior economic situation in the North would be disastrous for the people of Korea. Still worse, that would still be better than the alternative of their failure, which would invite even further Japanese reprisals, and considering that Japanese colonial habits by the early 1940s already included the assimilation of all Koreans into good little second-class Japanese, the elimination of the Korean language, the use of forced laborers and comfort women, and human experimentation for medical research, that does not bode well. As for the Home Isles, a Japan that makes no clean break with its past, limping from national trauma to national trauma...I highly doubt it would come off that much better.
As for whether the West would intervene to preserve Fascist Japan? Well, as I recall, the Americans played a key role in the death of Ngô Đình Diệm, in spite of the fact that he was their dog. When Syngman Rhee fell, they evacuated him from Seoul, but did absolutely nothing to restore him to power - the same goes for the assassination of the second major strongman of Korean politics, Park Chung-hee, an act which the US had little involvement with (unlike in Vietnam) but did not regret at all. When the Portuguese Estado Novo was toppled, the Americans practically cracked open a bottle of champagne in celebration of their decolonizationist policies being followed by yet another democratic government: as you yourself say, the US was no friend of colonial powers in general, even when those colonial powers opposed Communism; they much preferred to support domestic right-wing governments, and in fact had been funding Angolan rebels against Portugal since 1961. In Spain, though he had been useful after 1955, only a handful of people seriously mourned Francisco Franco or the end of his system of governance. In fact, let's take the Portuguese example one further - Angola's independence ended not in the victory of the American-backed rebels, but with the formation of a socialist single-party state. Mozambique declared independence as the People's Republic of Mozambique, which was still actually supported by the UK as a counterweight to Rhodesia, and the American CIA's attempts to support the right-wing rebel RENAMO were stymied not by foreign involvement, but by the US State Department itself, which flatly refused to recognize or deal with that particular terrorist group; RENAMO's support thus had to come primarily from apartheid states like South Africa and Rhodesia.
So, basically, likely a Communist united basket-case Korea, Communist united less-sane-than-historical,-but-still-more-sane-than-Korea Vietnam, and a basket-case Japan trying to put together a democratic or at least slightly less fascist government for the first time since their brief fling with it in the 1920s.