China and Russia have a very complex, layered relationship. On the positive side, they resolved their boundary issue a few years ago, the PRC might not even exist but for Soviet assistance after the end of World War II, and, above all, they have a common enemy: the US. On the negative side: there is plenty of racism at play, particularly on the Russian side; the Russians were enthusiastic imperialists in China at the end of the 19th Century; the relationship between the two countries was extremely frosty during the 1960’s; and the growing imbalance of power between the two does not bode well for the future.
As China rises, it will increasingly threaten to turn parts of Siberia into an economic colony, but Russia as a whole is much too large to be a vassal state. It is likely, in the long run, that the Russians will view the Chinese as a greater threat than the US.