Somebody coming at Moscow from the Siberian side will have a long trip, and a long logistics train. Then again, the same can be said about the Russian army trying to fight a Chinese invasion of Siberia. But I doubt they are going to invade Russian territory any time soon, not with Putin in charge there.
More likely candidates: grabbing Taiwan, conflict with Japan, and, lastly, doing something with Kazakhstan. Kazakhstan has lots of oil and mineral wealth, a small population (16M or so), is ethnically divided, and it's unlikely that anybody outside the Muslim world will ride to their defense.
You’re greatly underestimating the Borat factor.
Siberia all of the way to the Urals let's them move troops by foot if they have to and if they control Siberia all the way from the Urals China, they have plenty of resources and a defensable land mass worth the fight.
As to Vladimir using nukes.....maybe when the Chinese get within 50 miles of Moscow. I don't think he is as smart or tough as he likes to make out. Stalin would not have batted an eye.