21Vianet is the only Microsoft cloud provider allowed by the CCP. They are not Microsoft in anyway, shape, or form. In fact if you are a subscriber of their services there is a lag in updates.(presumably so the CCP can review first)
Rather, Microsoft has decided to outsource its Azure cloud operations in China completely due to government regulation. It wasn't that they had to have a partner, it was that they couldn't do it at all.
Microsoft still exists in China, they still have a few thousand employees there in sales and R&D that are entirely paid by Microsoft.