Chinese territorial expansion has involved continually annexing border states. It starts with a tributary state relationship, and the imposition of puppet rulers and ends with an invasion, when the puppet ruler decides he's not China's plaything. When successful, these invasions result in new provinces. We tend to assume that the Chinese are playing by the rules set by Western experiences up to and including WWII. I think they've been playing by Chinese rules all along, and have merely been biding their time. And the principal Chinese rule is that he who has the muscle makes the rules or as Thucydides put it: "Right, as the world goes, is only in question between equals in power, while the strong do what they can and the weak suffer what they must." Just because you've been nice to the dragon doesn't mean the dragon will respond in kind.
You got it right. This is a primary reason for conflicts between China and Japan going back many hundreds of years. China continually demanding tribute from Korea, and Japan intervening against China. (Japan also sought tribute from Korea.) China also wanted Japan as a tributary state, but Japan resisted. The nations who resisted, continued as independent states. China has never given up on swallowing Korea, Japan, Vietnam, etc.