That’s the same thing with every country, if you’re using VPN to access a website that the government has declared illegal then it is illegal. I don’t think VPN itself is what is illegal in China, it is what you’re using it for; if you use it for something illegal, then it is illegal.
It is not the logic of CCP.
In china, establishing international communication channels(VPN e.g.) without the permision from authority is illegal.
While China gov never declared twitter, facebook or bitcointalk illegal.They just block them using GFW and they never admit they block those sites.
But if anyone in China want to explore those sites without permision, one has to violate the law.