Of course, unlike true relics, the power of the rings is temporary. i.e. if a ringbearer is slain, as can happen, the ring is of no use to anyone else. Its lieutenant and the leader will know immediately of course, and the acquisition of a replacement vocator will be started. A new ring will be empowered instead and given to one of the many Vocators-in-training. 

We'd imagine that if there's anyone in the Empire that has proven to elude the vocators this long, he or she could easily be in possession of many such dis-empowered rings taken from those (s)he has slain. It is only the relic of the Vocator's leader that retains its power between owners. The reason it works to transfer between owners the way it does, is that all vocators change slightly upon being given a ring or necklace and slowly change further over time. Generally, by the time any of them try take on a superior, let alone the leader, they have changed sufficiently for the relic to consider them eligible.