Well, China has many times rejected the idea to form an union with outer Mongolia, due to their poverty and lack of resources.
Russia is an ally capable of independent economic strength and innovation, this makes it a premium asset to China, a type of asset even US doesn't clearly have today.
The regard and respect shown to Russia by China is beyond any other nation, and their return provided back is also beyond any other country. Russia enjoys a similar position in China's hierarchy as Israel enjoys in America's.
But in the medium term, I would guess that the vast majority in China and even many Russians living in Vladivostok would want the region to be returned to its original owner. Because China is simply much richer and many of the locals are already cross border migrant workers in the Harbin direction.
However, in my view, this land transfer should not be framed in anything close to resembling hostility to Russia. It should be done in such a way that existing Russians are allowed to have dual citizenship and a long adjustment period where they will have privileges/opportunities of both the Russian and Chinese system.
And Russia should be geopolitically compensated with expanded territory in Europe.