Cinsidering how China has a hard-on for "one China" and reclaiming "lost lands", I'd bet good money on China eyeing both Vladivostok and "outer Manchuria".
There was also that issue of the border along the Ussuri river.
China may be eyeing Vladivostok and a chunk of Siberia based on historical claims, but could Putin cede those and survive politically? The infosphere in Russia has many hard-liners who are already very critical of the Russian execution of ops in Ukraine, while still on board with the war aims of the Putin government in Ukraine ... for now. These are factors Putin must consider.
Should he make such huge concessions to the Chinese in pursuit of equipment for immediate war aims, I think -- and this is only my guess, nothing more -- that they'd turn on him for trading one part of Russia for what they see as another. In domestic Russia, wouldn't it look like a wash? Or even a tacit admission of subservience?
The various border clashes Russia and China have had also underline the fact that while they may have made a production-number out of "unlimited support" in the February summit, both sides may actually be approaching this from a perspective of
Realpolitik or quid pro quo. That's what I think such a trade of territory for Chinese support would boil down to, and again, could Putin survive it? It would essentially be acknowledging Russian subservience to China, and that would not be a good look, politically, in any authoritarian country, especially one already experiencing internal dissent over a failing war.
That's my read on it, at least. Happy to read any rebuttals or other differing views, of course. It's a complex issue and no one here (and that includes me too) can be sure, in my opinion.