Japan wouldn't just give into China, no matter how many nukes it pointed at it. Japan and China have hell of a lot of bad blood between them. More so then China and the USSR.
Japan wouldn't make a good jumping off point to invade Alaska because a large naval force would still be needed to move troops and supplies from the Northern most Japanese Islands to Alaska. There is thousands of miles between. Hokkaido to Alaska, 4,500+ miles.
While the distance from Siberia to Alaska is only 51 miles. You wouldn't need a massive navy to move troops and supplies to and from Alaska.
We know Europe was falling apart. Which means the Soviet Block countries would be in chaos. The USSR would be putting it's resources into holding itself together and fighting other European nations. Siberia only has about 25% or so of Russia's population. But it makes up about 80% of Russia's size. It wouldn't be worth defending the whole area. When it means going to war on three fronts.
Think about it. The USSR would already be at war in Europe, trying to keep the USSR together and then would have to go to war against China which is a greater Super power then the USSR. It would mean the end of the USSR if that happened.
It would make sense for the USSR to just accept China's annexation of Siberia at least part of it and concentrate on the European front. It is also likely that the two powers agreed to a deal. China gets to move through the USSR in exchange for any oil China captures.
And yes Siberia has a lot of resources but it's in Siberia. Very hard to get to and hard to extract. It's also possible that in the Fallout Universe they never discovered a lot of resources. There's also a lot of oil and other resources closer to Moscow. Such as the Caucasus. Also in Poland and Ukraine which the USSR would be fighting to keep during the Resource Wars.