As talks between Kim and Trump march forward, China is resigned to the new situation at its eastern border and is focusing on other areas, specifically trade. In truth, China's main trade opponent is not the US, but Vietnam.
Vietnam's main edge in trade will be that it is less expensive. Vietnam is, in many ways, less developed, yet more free to be expressive. Hanoi doesn't sanction the same censorship as Beijing does. Many hard-working Vietnamese are hungry, even desperate for income. A hard-working, uncensored, hungry, less-expensive people will be difficult for China to compete with on many fronts. This is entirely beside any point about political tension between China and Vietnam.
The meeting between Kim and Trump is less-than-satisfactorily explained. Suddenly they want to talk? Some "teamwork" consultant trying to sell a book will likely attribute it all to diplomacy, along with the preemptive speculation that Kim would give up the nukes because he got them. More is going on behind the scenes and if the true story is ever told it may not be told for ten or twenty years.
As for the Western spin about China's constitutional changes, it is all about the party, not about Xi. The humble pig farm worker, Xi Jinping, did not rise to power by publicly trying to serve himself. He has followed Robert Greene's 48 Laws of Power to a tee and will continue to do so—that means putting the party first in his public agenda.