I've traded into Vistana 2 times. Each time, I use the Diamond lane and request top floor in a bldg w/ elevators. If you have a ressie for a 2 bdrm slps 8, you have a great chance of getting Cascades, unless they've changed their policy since Feb 03.
Also, after emailing my request, I have called both times. I get the same person (a nice lady who's name escapes me). I think she's the only phone support for Diamond lane. I found that she responded quite well to me. I made sure to be a happy, non-pushy customer. Not only did I get to confirm that they had received my request, it worked! Last time I had 4 units reserved and we all got Cascades (diff bldgs) for Presidents week, no less!
Good luck!