The Peking Convention is a decision available to 
Russia regarding 
Manchuria, owned by 
China.
The requirement is that Russia is a great power and have sphered Manchuria. It gives 2 prestige and a core on Manchuria owned Kyzyl (Outer Manchuria). It also gives China the event The Convention of Peking, which usually leads to the annexation of Outer Manchuria.
It is not possible to take, if Manchuria does not own Vladivostok, so if China westernizes and annexes the substates, then it is not possible to take.
The ingame text states: Since the treaty of Nerchinsk (1689), the Russian Empire has recognized the Argun river and the Stanovoy mountains as the natural frontiers between Russia and China. In the mid 19th century the Russians would expand beyond this border to seize the vast lands between the Stanovoy Mountains and the Amur River. The Western Aalies would later bring the Second Opium War to a conclusion by burning the Chinese Emperor's summer palaces.
It leads directly to the Russian decision The Ruler of the East
Vanilla
In Vanilla the Chinese substate system does not exist, and therefore Russia has to sphere either China or an independent Manchukuo