The very definition of mandatory and voluntary is the presence of that agreement in the CC&Rs. There is no real definition in writing, my understanding is that they are internal terms to Vistana. Mandatory means that if you own at that resort, you must be a member of VSN. Voluntary means you can opt in or out of the club and with that Vistana can opt you out based on certain terms. Those terms are currently if you buy resale, you are opted out.
If they change the club rules, then the terms in that CC&R change with it. They can change what the benefits of VSN are and point values, but if they do that it impacts that agreement in the CC&Rs along with it. All mandatory resorts have something in common and it is that agreement in the underlying documents. If Vistana could get rid of mandatory resale, they would have done it long ago. The problem is that they can't, because changing the CC&Rs would require a supermajority vote of the ownership.