What if the US gains control of parts of Siberia via the Japanese?
Russo-Japanese war, WWII, alternate WWI, or some other incident could all work for getting a large chunk under Japanese control. At that point it's not hard to fathom conflict with the US. All you need is for the US to win and for Russia or its analog to not have the strength to force the return of it's previous territories.
While the Japanese getting the place is rather more plausible than the Americans getting it, none of these three events is actually a good chance for Japan in anything resembling OTL:
1) Russo-Japanese War: Everyone thought Japan would lose, inluding her sponsor, Britain. This was partly fixed notions of "primitive Asiatics"... and partly because the Japanese were both absurdly skilled and absurdly lucky at pretty much every stage of the war, and should really have lost. And yet despite their winning streak, they still managed to end the war exhausted enough to take outside mediation because their finances were stretched to the limit and their casualties were unsustainable. For their trouble they received half of a largely useless island, a confirmation of their control of Korea which already basically existed, and some nice but not overwhelming advantages in Manchuria.
For the Japanese to physically take Vladivostok would be crazy. For them to keep it would be crazier: Russia will not accept that loss, and Russia, as the Japanese knew, simply had enough men to keep fighting for as long as she liked. And Britain (and all the other powers) would tell Japan to cool it the moment she went into Siberia.
2) WW1: As we saw OTL, Russia completely came apart for two years, and the Japanese had ample time to occupy Vladivostok and establish a servile government. What they didn't have was the necessary manpower and politicaly will to fight a massive and probably futile land war against the re-assertive Russian government on its home ground.
Remember that the Russians have handily won each battle in that part of the world since 1905. Japan's army was always a very scantly-equipped force by European standards, and in 1922-3 the Red Army was not a slouch: it had just fought and won the RCW and its equipment wasn't quite as outrageously obsolete as it would be later in the decade. And the Japanese armies were hardly modern either.
If the Japanese were really determined, it might be possible for them to knock the local Whites into some kind of order and hold a line around Baikal, which is the only real natural defence for miles and miles and miles; but that state would be large enough to be a puppet of Tokyo but certainly not a colony. Why would such a state (a pretty hard-right one, we can be fairly certain) ever want to join America?
And you'd need to change Japan quite a bit. I think the furthest they ever went OTL was a brief stay in Chita.
3) WW2: The Japanese managed to draw a skirmish in 1938, lose a skirmish in a most humiliating way in 1939, and lose all Machuria, an area the size of western Europe, in eleven days in 1945. This speaks ill of their capability to occupy much of Siberia.
The fact is, Japan had a fairly backward army lacking the resources necessary for modern industrial warfighting. It never supressed the Chinese, despite their chronic inefficiency, and the whole Pacific War was contested by a small number of their most elite units agianst very minimal enemy forces in climates were modern industrial warfare was impossible.
Local Japanese aggressions (Khasan, Khalkin Gol) got nowhere. A large, co-ordinated attack on the Soviet forces in the Far East would be unlikley to get so very much further (Vladivostok was a pretty fearsome fortification at the time: better than Sevastopol', Soviet strategists reckoned, and just ask the Germans about Sevastopol'), and would end up with the industrial Soviet war machine hitting Manchuria like the proverbial ton of bricks. Sure, the Soviets had an obsolete and scatterbrained force in this timeframe compared to the Germans, but Germans the Japanese were not. The pre-Winter War Soviets ran rings around them, and they are not going to tolerate a major Japanese attack.
If the Japanese attack when the Soviets have all their resources committed to fighting Germany... this does fuck all. The Soviets can if necessary evacuate everything east of Irkutsk, blow up the railways, and let the Japanese have fun
walking the thousands of frozen miles to any target of strategic value: the evacuated industries were in the Urals and Kazakhstan,
even further out of Japan's reach than they were out of Germany's. Japan's situation will only worsen when the failure to attack the Southern Resource Area (and if she does attack south, where does she get the troops to go north from?) results in a major supply crunch.
And then Germany surrenders and the fun begins...