Here's an update on my access experience as a dual MVC-Vistana owner, in case it helps someone.
On the Vistana.com side I could convert to Abound (which I did) and my account showed Executive status (3*) but checking my Club Point balance or trying to make a Club Point reservation both yielded error messages once the pages I was redirected to loaded on MVC's website. On the MVC website account side, reservations worked and I could see my MVC Club Point balance but not my Vistana elected Club Points. Also my Executive status from the Vistana side was not being applied to my MVC account, leaving me down at Owner/Member level. Clearly something was not linked up!
After many phone calls with well-meaning reps with Vistana and MVC, we figured out half the problem. Before I became a MVC owner earlier this year, I had made and rescinded a MVC purchase in 2019. During that process MVC created an owner account for me and left it floating around the system after rescission rather than closing it. Instead of using that existing owner account this year when I bought, they created a second owner account and put my MVC points in there. However as part of the Vistana system merger, they associated my Vistana account and its Executive status with that abandon MVC owner account. It took 10 minutes for MVC to merge the two MVC owner accounts into the newer one with the points and then my MVC account finally showed Executive. The rep said she could now also see my VSN elected Club Points sitting in my MVC account next to my MVC points. However that status update came at a price as now I can't view my Club Point balance at all in my MVC account, same as when I click through Vistana.com. The only way to see my CP balance is to make a reservation through my MVC account and when it gets to the point allocation page, it displays all of my MVC and VSN elected CP balances. Progress, at least! I used those VSN elected points in combination with my MVC to make reservations through my MVC account. When I tried to make a CP reservation via Vistana.com, the reservation tool was working for me now but it gave me a message that indicated that I could only book reservations using my VSN elected CP alone (i.e. that method of access is not combining with existing MVC CP yet). Very interesting!
So it appears that the MVC system Vacation Club Point balance page can't yet handle displaying VSN elected points (for some users at least) which is why is crashes out with an "Apologies" message regardless of whether I access it from Vistana.com or the MVC website.