I'm not sure I'm interpreting your question accurately, but based on your example, if I'm interpreting it correctly, the lower level owners who don't own enough points for the 13-month booking window might actually be impacted less (or at least no differently) than those higher level owners who regularly book DC points at 13 months. MVC only releases up to 50% of the inventory at 13 months, so those Vistana owners who would wind up with lots of DC points (in a theoretical membership in the DC) would be competing with other high-point MVC owners for that 13-month inventory (but then, presumably, their Vistana ownership would have also been deposited to the DC Exchange, so that might siphon off some of the demand for the MVC inventory). At the 12 months point, a whole new batch of inventory is released that the lower point owners can still access as always. Yes, anyone not confirmed out of 13-month inventory would be competing for that new inventory drop as well, but that's the same way it is today.
Obviously if the Vistana owners only deposit low demand weeks into the DC Exchange but chase high demand intervals, that creates an imbalance, but that's the same way it is today if MVC owners at lower demand resorts in New Jersey or Branson convert to points and try to book Hawaii. They are creating more demand for Hawaii (they might have to bank/borrow and use three years of their points to book the high demand stuff, though), but the stuff they deposit is not as demanded.
And remember Select, Executive, Presidential, and Chairman can all book at 13 months. The only owner level that is different is the Standard Owner level - and even they can book at 13 months as well, it just costs a 20% point premium that the higher levels don't have to pay.