You need to pay for II membership, annual fee is around $89 per year, with discount if you buy multi-year membership. Each trade fee varies, from $199 when trading into non-Vistana brand resort, to $149 (I think) for another Vistana resort.
If you currently have RCI, are you looking at II to increase your trading options? Do you not find Mexico or Hawaii resorts in RCI which you may like to trade to?