I'm not sure I totally agree with spiceycat's explanation. There may have been some discontent from Sheraton, but as I remember it, the various Homeowners Associates voted about whether they wanted to stay with RCI, go to II exclusively, or affiliate with both. We owned in the Fountains only at the time, where the decision was to offer the flexibility of using both companies.
I certainly was not privy to any of the discussions leading up to the votes, so there may be some truth that Sheraton was unhappy with RCI. I just can't say.
Vistana is readily available in off season months such as September. The availability is limited, on both RCI and II, for peak times such as around Christmas and Spring break. Many RCI owners are also switching to RCI Points because Vistana is a cheap entry into the Points system. That may also have some affects on availability.
What time of year are you trying to travel?
Sheila