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...
I think the hundreds of F-35s of US, Japan, Australia, and SK are the true threat that China is preparing for. These are the clear and present danger to China in the immediate future. The 24 Russian Checkmates to Vietnam is still hypothetical, and is most probably still decades away from happening.
So, if China has to prepare for a fight against massed F-35s formations from Japan, USA, SK, Australia, etc. Shouldn't China be more than prepared for less than a hundred Checkmates flown by Vietnam and India? In any case, Russia had already attempted to sell Su-57s to India. But that has so far not materialized. Thus no speculative military deal is set in stone. There were many speculated deals didn't happen in the past. Such as the: early model J-10 sales to Pakistan, J-10 sales to Iran, or Tu-22M to China, etc.