What is the ruling on eating a fish and chips meal at McDonald's restaurants in China if it is said that the oil used is vegetable oil, while there is no certainty about that?

If you do not know that the oil used to fry fish is derived from a forbidden animal or from an animal that was not ritually slaughtered, then eating this fish is not forbidden. This is because the default ruling for things is permissibility, especially if you are told that it is vegetable oil. It is unlikely that a well-known restaurant would lie about such a matter, being keen on its reputation.