That's really a shame. It's a lovely resort but you clearly bought for the wrong reasons. So I can understand your disappointment and frustration. Maybe this will help.
First, Sheraton Vistana Villages was dual affiliated until recently. There are still some owners who are able to trade via II. Check your purchase contract for info about exchange company affiliation. Then call II directly and ask if you are able to trade through them.
However, II usually restricts people from trading from one Orlando area resort into another so that probably wouldn't do you much good for trading to DVC anyhow.
Excess rental inventory is indeed available to all timeshare owners who are members of either RCI or II. The prices are usually higher than you were quoted - until a couple weeks prior to travel, if inventory is still vacant. Sometimes, cancellations are seen at excellent locations.
By making the effort to join one or both of these exchange companies and searching online at RCI/II regularly (with patience), you really could get great results!
You really need to consider joining
TUG - Timeshare Users Group. Go there and read "Advice" articles on how to make the best use of your timeshare. Learn how to use your ownership to best advantage by reading the forums (like DIS).
TUG membership would enable you to access independent resort reviews & ratings for over 2,000 timeshare resorts. It's $15/first year and money very well spent. We always read reviews prior to accepting an exchange. Please use my name (Lisa Potter) as a referral if you join TUG - it extends my TUG membership.
I don't work in the timeshare industry, just enjoy our family's timeshare vacations. I think it's a terrible shame that you've purchased as you have. Since it's too late to rescind, it's going to be important to chalk this up as an expensive mistake. Then focus on how to get the most out of what you have - and enjoy it!!! Wish you well!