I would say that even in as little as 100 years, it would be far more profitable for China to start off-planet colonies than to try to annex a neighbor.
Well, Lake Baikal in Khavbarosk near the Amur contains 25% of the world's unfrozen fresh water system, which a simple water pipeline can connect and quench the arid northern Chinese provinces, but that might cause an ecosystem disaster akin to the depletion of Aral Sea during USSR era. Such a solution would be far less costly than the South-North river diversion project, which is tantamount to declaration of war on India if the Brahmaputra was 20-30% below water level than it is today
I'm sure the Russian capacity to share resources (ie. allow Chinese companies to access resources in Siberia) is much greater than India's Brahmaputra, which supports untold hundreds of millions of people.
I wouldn't be surprised if Lake Baikal will be to China, what the Aral Sea was to USSR, esp. given China's acute drought and water shortages in the Northern Chinese provinces. This is even more important as shale gas (fracking process) and even coal demands huge volumes of water, so if China ever wants energy independence with shale gas, it needs LOTS more water, and Lake Baikal just so happens to have LOTS of water.