first of all, concerning China's representation in EUII, you must not forget that the core of the game are the european countries and those that had strong interactions with them ( ie OE, Persia, Khanates ). therefore China was not intended to be fully modelled, same goes for the very misrepresented situation of the Indian penisula throughout the game. This is not a problem to me because I admit that it is not the focus of this game, these countries have been made playables as a courtesy to the buyers and a commercial boost, but don't forget the game's title, "europa Universalis".
Furthermore, China had very few interractions with Europe during the time frame, so its influence on the european centered game is minimal, therefore is not a priority. Feel free to do a mod of your own and have the community enjoy it.
As for China in Vic, there it played a truly inmense role, being the pilar of the western powers action in far east asia at the time. So here, failing to correctly implement the features needed to render the situation properly would be a setback ( although like many things in EUII, the community will probably account for a great part of the game in later mods ).
from what I have understood, provinces will be the small military territorial divisions, while regions will be for administrative /economic purposes. At the end of a war you will get provinces in the peace treaty provided that that is wath you were going for. I think there should be some form of trade tech that models westerners trade superiority,and allow the one province you control to drag the trade of the surrounding ones, the extent of the dragging depending on the level of trade opening forcefully imposed on the target country.
Say that China is divided in 20-25 regions, like the provinces in EUII, which represents a total of say 200-300 provinces. The western country wouldn't have to annex big chunks of the territory as it would only give them huge BB. just annex one lousy province, and you could drag to it, depending on your trade tech and that of your target and the level of trade agreement reached, 25; 40; 60 or 75 % ( these numbers are just out of the blue examples ) of the trade wealth of the surrounding 20 to 75 provinces. this would represent the likes of Hongkong and the commercial carving up of China. Add to that the possibility to build TPs in already controled provinces and you have tools that could easely represent most of what was going on not only in China but also in the dying OE, and what could have happened in South America had it not been for the Monroe doctrine ( but then, you can change that in game).
a british trade agreement with Argentina giving Brits the right to build up TPs throughout the country would represent Queen Victoria's saying that Argentina was the most profitable colony of the crown.
Altough this raises the issue of commercial pillaging of a country ( like in China's case) and mutually benefiting trade agreement like in the case of Argentina and UK. How to model that ?
PS : Could you believe that the nowadays bankrupt country that is Argentina was in 1920, just after WWI the 6th economic power in the world ! Yes believe me, if you go to buenos Aires you feel like visiting Milano turned poor. Argentina and China are indeed very good tests for Vic's trade model.