I don’t see this as likely. The imbalance would be so great there’d be so many Vistana owners unable to book what they own, leading to a PR nightmare and likely litigation.
I've been presuming, without evidence of course, that Marriott owners will only have access to inventory from Vistana owners who opt-in to DC. So those opted-in weeks or SOs become part of the DC pool, at least for that year, and then Marriott owners can try for them.
For the reasons you state, I can't believe they will open up all of the traditional VSN inventory to owners who are not part of VSN. That just doesn't feel like something they could get away with.
For people who do not elect to opt-in to DC, they will continue to exchange within the VSN pool, which may shrink in both supply and demand, which shouldn't be apparent to those who use it, unless MVC manages to starve it of inventory to the point that it loses viability as an exchange network. Again, this is my best guess. The proof will be in the details which will be revealed on March 14. Oops, I mean March 21. Scratch that, maybe March 24.