Honestly, I really do not see how VISTANA could control which units the owners decided to bank with I.I. or R.C.I. etc. And, I too can tell you that with RCI you can be placed in any section.
What determines your location seems to be TWO things.
1. What type of unit... They do have to guarantee you a unit with the number of bedrooms and beds that you have exchanged for.
2. Your check-in day. At Vistana they have sectioned out the units into Friday check-ins, Saturday check-ins, and Sunday Check-ins. These are colorcoded on the resort map on the Vistana site.
Shortly before check-in day, they start to assign units. And, when you check-in, they will place you in a unit that matches your unit-size requirements, and is designated for that days check-in rotation.
We were recently there, through RCI, and we were VERY lucky. We were given what I would call a 'penthouse' unit on a top floor of Cascades, overlooking the pool. (the view without the noise and the loss of privacy!!!).
I would definately contact I.I. and Vistana (thru Diamond Lane???) to get the low-down. I would be very surprised if they could guarantee these sections.
PS: Even if you did get one of the other sections, as long as you get a nice unit which has been maintained and renovated, you should really love Vistana!