Historically there was a major war scare in the Kremlin that Japan would attack the USSR in 1937, but nothing came of it because Japan invaded China instead. In the late '30s neither country was really mobilized to support a protracted total war with the other; had Japan committed most or all of the troops that it did against China in 1939 to a war against the Soviets, say, after Khalkhin Gol, plus those available in Korea and Manchuria, there would have been about 35 divisions plus other units (Independent Brigades, Regiments, etc.) adding up to roughly 1.2 million soldiers, 3,500 or so field guns, 2,000 tanks, and 1,800 aircraft according to Soviet data.
Although the local Red Army forces in Far Eastern Russia were at the time considerably smaller than the above, the Japanese figures account for the vast majority of their standing Army in 1939; IGHQ would have only had about 550,000 men, 1,000 field guns, 300 tanks, and 1,200 aircraft on hand elsewhere, whereas the Soviets could call on tremendous reserves from European Russia to deploy to the Far East. Even if they took Vladivostok and Primorye they would be looking at a war of attrition in the Trans-Baikal against an undistracted USSR with a much larger standing force
before the Japanese 1938-41 four year industrial plan could take effect. To decisively defeat the Soviet Union in a
one-on-one confrontation (i.e, defeat the Red Army and occupy all of Siberia to Lake Baikal) it was estimated that they would have to wait until 1943 (Hachi-Go plan).
When Germany attacked the USSR in 1941, though, it led the Army General Staff to draft (and partially implement) a truncated version of the 1940 War Plan (itself similar to Hachi-Go Concept A), referred to as the Kantokuen Plan (article here:
https://en.wikipedia.org/wiki/Kantokuen). With no US sanctions to worry about Japan might have gone through with it - though likely not without economic fallout with the Americans for being so blatantly in bed with Hitler.