Friday you get put in Spas and Sunday you are sent to Falls. On Saturday it is the other sections - try to get in Lakes or Cascades because they are the newest, although I was put in Falls (the second oldest), which was beautiful.
I requested Lakes and Cascades three times (twice by phone and once by e-mail) to no avail. You can only request and hope - if your room is no good then really kick up a fuss and they will move you. But don't get stressed about it - I was so worried about getting put in Falls and it turned out to be one of the nicest and quietiest parts of the resort.
As for a tip - go to the Vistana website and fill in your details via the diamond lane check in. When you arrive you'll miss the main queue and get checked in almost immediately!