China will likely be more focused on extending influence through Asia and Africa and making satellite nations of former soviet republics. I think they'd gain little from direct conflict with the US in the Pacific but I suspect you'll see a lot of military buildup in the Pacific especially along the Ryukyu islands and potentially the Kurils in such a situation. If Moscow totally collapses the far west of Russia isn't exactly in a position where they can be independent and they'll likely have to rely on alliances with a different power. Their choices being essentially China or America by proxy of Japan.

Places like Sakhalin and potentially even Kamchatka might see potential alliances with the US sphere is more beneficial. Especially once we consider that overland transportation in the region is virtually non-existent and they rely on naval trade which the US largely dominates in naval capacity compared to China.

Places like Vladivostok or the Jewish Autonomous Oblast I think are more likely to get dominated by Chinese interests in such a scenario.

However I don't really see a "total Russian collapse" scenario as realistic. It's far more likely that Russia simply becomes less relevant as a world power and increasingly beholden to follow the Chinese sphere of influence. I suspect a kind of Neo-Cold war with a number of proxy wars on the fringes to be the most likely scenario. Similar to Orwell's "The Theory and Practice of Oligarchical Collectivism" albeit with a dipole quasi-stable mutually beneficial conflict state rather than the tripole one he envisioned. Unless we count neutrally aligned states.