One idea: some high-profile Korean communists befriend the Soviet leadership, and thus the USSR has more pro-Korean policies. Maybe
Kim Tu-bong moves his
Korean independence movement's government in exile to Vladivostok following the end of the Russian Civil War, and becomes friends with the Soviet leadership. They convince the Soviet government not to stop the
Korean immigration to the Far East, and even support it via the establishment of a Korean ASSR (or even Korean SSR) in
Primorsky Krai, as part of the
Korenizatsiya movement.. Instead of viewing the Koreans as suspicious due to their ties to the Japanese Empire, the USSR views their Korean ASSR as a step towards liberating the whole of Korea (similar to their use of the Moldavian ASSR to justify their claims to all of Moldavia).
In the interwar period, the Pimorsky Krai region (now Korean SSR) continues to take in a lot of Korean immigration opposed to Japanese rule, and these immigrants are therefore mostly pro-Soviet (as their new government in the USSR, unlike Japan, allows them to use their own language, etc)
WW2 happens much as in OTL; following WW2, the USSR lets the Korean SSR become officially independent (with its capital in Vladivostok, now renamed Sinhanchon). The USSR allows it because that region is mostly Korean at this point, and clearly very loyal to the USSR regardless, to they decide to officially make it independent (though nothing much changes de facto) in an attempt to garner more support in the upcoming Korean War.
Korean War happens; Korea is partitioned similarly to in OTL. The Democratic People's Republic of Korea aka North Korea (a Soviet puppet) keeps its capital in Vladivostok/Sinhanchon, because of its existing role as the capital of the Korean SSR, and because they didn't manage to gain control of Seoul. (though they officially claim Seoul as their capital)
Sinhanchon (Vladivostok) performs the same role as the North Korean capital that Pyongyang did IOTL; Pyongyang stays much smaller (less than a million people) whereas Sinhanchon, by 1990, has about 3 million people.
ITTL, North Korea doesn't end up with Juche, instead staying much closer to the Soviet system, due to the countries' strong ties. (And in fact, Kim Il-Sung never even becomes its leader- Kim Tu-Bong is the first North Korean leader). Following the collapse of the Soviet bloc in the 1990s, the two Koreas successfully agree to merge along the German model. One of North Korea's conditions of the merger is that while Seoul remains the ceremonial capital, the center of government is to be located in Sinhanchon. (this is because South Korea's economy is stronger, so North Korea was worried about being left behind, and wants to ensure that the country as a whole has more focus on the North).
The newly-reunited and capitalist Republic of Korea experiences rapid economic growth, and its capital of Sinhanchon in particular grows a lot. By 2020, the population of Sinhanchon (including its metropolitan area) has reached around 8 million people.