Was your existing Vistana ownership purchased resale or direct? If you are already an SVN member, a Flex purchase won't get you anything you don't already have except for the ability to book the 5 Flex resorts 12 months prior to arrival. Most of those resorts don't have enough demand for that priority access to matter much.
The salespeople showing you Marriott resorts are speculating. Hopefully they weren't showing you Marriott timeshares, because Marriott Vacation Club is a separate company having nothing to do with the Starwood-Marriott merger. Not to mention, the Starwood timeshare business is being spun off and sold to Interval Leisure Group (ILG) prior to that merger.
It is possible -- likely, even -- that Starwood/ILG/SVN timeshare owners will eventually be able to convert to some flavor of "Marriott" points system to access both Starwood and Marriott branded hotels and [non-timeshare] resorts; however, that is years away and subject to lots and lots of decisions which have yet to be made.
If your travel priorities are to go to different Starwood timeshare resorts, the only thing I would recommend doing would be to purchase a resale WKV Platinum Plus or a mandatory SVV property. Use those StarOptions to travel to the different SVN resorts. Don't let anything having to do with Marriott factor into your decision, and certainly don't purchase anything directly from the developer unless you know exactly what you are doing and why. In most cases, it's not the right decision.
. Would be giving up 20 weeks at wlm
