Queen Elizabeth II is a member of the Order of the Golden Fleece. Does that make her also King Felipe VI's "vassal" per your logic?
Changing subjects, I wonder what criteria are used to invite non-reigning RFs. Why are invitations extended to the RFs of Greece, Romania and Serbia, but not of, let's say, Austria-Hungary, Italy, or Portugal? I would understand an exception for King Constantine since he is a living former King, but what about the others? Was the criterion being related to Prince Philip?