So Boerd said
You way overestimate China and Japan. The Russian Far East is the finest territory for guerilla operations in the whole world. Invading and defeating Russia from the east is impossible.
Basically this. I don't think that China or Japan can do anything else aside from having taken Vladivostok. I mean, are they going to force march through Siberia? They'd basically kill their soldiers so they can say they have control of basically nothing.