Sure, if China has more vassals that can replace Russia or even ones that are aligned against Russia (like Japan) after solving the US problem, China can indeed do what you describe and turn her economic, covert and military guns to blast Russia into pieces, retaking the Far East.
But this is not a moral action, without even trying to negotiate and straight up turning your (covert or overt) weapons on people who have fought and bled on your side to bring your empire to fruition just because some of them happen to be living on what was once Chinese land isn't the behavior of a sane or reasonable superpower. It is like what Israel does with their allies.
Why do you want to do this instead of just approaching Russians with negotiation? Having such large parts of the Far East is not necessary, not when China has resource rights in the rest. Arguably only Haishenwai is important, and only then once the PLAN is so large it can dominate the north Pacific.