To guarantee the same room and no "check in and out" process, you need to book the cash stay through MS. Of course, you may be able to get a better rate from CRO. You have to decide for yourself if the rate difference (if any) is worth the risk.
Be sure that CRO notes your
DVC confirmation number on the CRO cash reservation and that MS notes the CRO confirmation number on the DVC reservation. A request to stay in the same villa needs to be on both reservations as well. Doesn't hurt to repeat the request at check in.
Best wishes-