The way Vistana is still pushing these Flex programs makes me wonder if MVC will keep the 2 systems. Perhaps there will be an umbrella program over both systems that allow people to exchange to/from both systems.
Perhaps, but that particular move would require Vistana to create and manage a third inventory pool -- one for WestinFlex, one for SheratonFlex, and one for those who enrolled in the umbrella program covering them both. Because those who don't enroll (i.e. those who choose not to pay an enrollment fee) are still legally entitled to a proportionate amount of the WestinFlex or SheratonFlex units (i.e. the units that were not enrolled). And remember that WestinFlex and SheratonFlex are already umbrella programs including both points owners and weeks owners who transferred their units at resorts with underlying weeks.
Giving the multiplicity of Vistana programs -- WestinFlex, SheratonFlex, HomeOptions, mandatory units, voluntary units purchased developer, voluntary units purchased on the secondary market, Adventuras, and the Florida Club -- a Marriott-wide umbrella program will be devilishly complicated. I suppose if they actually create one, the arbitrage possibilities resulting from any new program will be fascinating (and will be analyzed to death here).