Unfortunately, due to GDPR legislation it's no longer possible to work out the VIN from the VRM using public websites.
If you're buying a car, you can ask the seller to provide you with the VIN. Alternatively, you may get lucky and one of the members on here who has dealer-level access to the MB DSB can convert the VRM to VIN for you.
An MB dealer might or might not give you this information, depending on how well their staff is training in GDPR.