MB may or may not be on the Vidafone network in the UK depending on the current situation.
Technically speaking, GSM devices can function even without a SIM card; in fact, you can use a SIM-free phone (but only to dial emergency services) and still make GSM voice calls. Mobile phone companies won't let your device use their network to make calls unless they have the correct billing information, which is stored in the SIM card.
So, the subscriber data is hardcoded into the communications module and identifiable by the vehicle's VIN; a SIM card is thus unnecessary for the vehicle. In the same vein, the idea of contemporary "eSims" is to store subscriber data on the phone rather than the SIM card.
Before you can pay for services, you'll need to register your vehicle's VIN and create an account with Mervedes me online. At that point, you'll be able to view all of the car-related services that are for sale. Subscriptions will be required for all of these.