You can use your timeshare to exchange for other timeshares with II. Unless you want to trade right now, there is no reason to join right away, unless you want to trade your 2017 or 2018 week.
With II, you would have priority to trade for other Vistana timeshares, but no guarantees. Vistana primarily deposits off-season weeks for exchange, so it depends on where you want to go, how far in advence you can plan, and how flexible your dates are. The easiest times to get an exchange at most locations is the fall - and summer in the desert.
The best way to get an exchange in to put in an on-going search at least 12 mos. in advance - this is like wait listing.
There is a yearly fee to join, and a fee for each exchange which you have to pay when you put in the request.