VOR is a special case. Due to its historical role, all IFR aircraft is assumed to have at least one VOR receiver
The fact that a VOR receiver is required on every IFR airworthy aircraft, doesn't answer the question. The question was: is it legal to execute the approach if the VOR (the ground station or the VOR receiver)is inoperative (let's say you took off with a perfectly functioning VOR receiver, but enroute it, or the ground station stopped working)?.
Could you still, from the legal perspective, execute the approach in the example (the EHAM ILS 18C for example)?