If Vietnam purchase 24 checkmates fighters from Russia, then it's "Checkmate" game over for China in SCS as far as 4 or 4.5 gen fighters go. No amount of J-11Bs or Su-27s or J-15s currently stationed on SCS islands can defeat 24 Vietnamese Checkmates.
China needs to dedicate an entire EMAL- CATOBAR worth of J-35s just to counter the Vietnamese threat in SCS. J-35s can't even be deployed effectively on Liaoning or Shandong and J-15s would get wiped out by the Checkmates. J-20s from mainland likely can't sortie to Spratlys without refueling tankers.
So while I'm glad Russia can profit a few hundred million from the Checkmate sales to Vietnam, China must dedicate an entire $12 billion CATOBAR (or two) just to counter the Checkmates from Vietnam.
That's a ton of Chinese resources redirected from facing US Navy into facing a tiny nation on the periphery, esp. if Checkmates are purchased in meaningful numbers. Every CATOBAR counts against the US Navy...