We have stayed at the Sheraton Vistana Resort as both "hotel guests" (via a Jetblue getaways package) and as timeshare owners. Never a resort fee, never a parking fee, never an extra cost for basic internet.
It might be helpful to know what website you are looking at that lists the $20 nightly fee. I am guessing it might be a charge levied by the 3rd party site that's showing the listing, rather than a fee applied by the Vistana itself.