You've got a few options.
[*]Dreams Unlimited Travel is a DISboards sponsor and almost always has an active link for Sheraton Vistana Resort at the top of this thread
- Direct from Sheraton.com
- Any trusted TA
- Redweek.com
- tug2.net (timeshare users group)
- eBay
The first three options allow you to book more like a hotel with variable checkin days, variable length of stay, cancellation options. The last 3 options allow you to book from resort owners, but you will need to pay up front, non-refundable, and usually only full weeks with Fri/Sat/Sun checkin. I'd be very careful if buying a week's stay on eBay.
Timeshare owners can book Vistana resort through either RCI or II as well.