Lool Dude the dispute of that barren rock is between you and Japan. If China gave up its claims and recognised Japan sovereignty over that barren rock relations with Japan would have been totally different from what they are today , since that disputes creates alot of enmity between both sides and stokes nationalism and hatred often between both sides. For one Japan wouldn't have been overtly hostile to China today and wouldn't join any alliances against China (especially not openly).
This has nothing to do with the US. And no Japaan is not a vassal of the US per se. Else they would do everything the US tell them and they would never have joined the RCEP in the first place since the US was vehemently against them joining. So you using the word vassal her is deceiving and oversimplification of what it really is.
If you guys can be so lenient with Russia who has caused far more long term loss of vast territory permanently from China, then why can't you let go of a small barren rock to have good relations with a far important economic and technology power like Japan?
If I was a Chinese leader that will be a logical thing to do. Lol Nothing much to lose, more to gain.