For the crust: Line a 9-by-13-inch baking dish with parchment, leaving a 2-inch overhang for easy removal.
In the bowl of a food processor, add the vanilla wafers, butter, granulated sugar, zest and a pinch of salt. Pulse until the mixture is a very fine crumb and is starting to clump together. Press the mixture into the prepared baking dish. Chill the dish in the freezer while you make the filling.
For the filling: Add the cream cheese, condensed milk, confectioners' sugar, grapefruit zest, lemon zest and orange zest to a large bowl. Whip together with an electric hand mixer until completely combined and there are no clumps of cream cheese. Fold in the whipped cream. Remove the crust from the freezer and pour in the filling, using a spatula to smooth it into an even layer. Return to the freezer and freeze for at least 4 hours or overnight.
For the topping: When the filling is set, remove the whole bar from the baking dish and place on a cutting board; removing the parchment. Decorate the top with the orange, grapefruit and lemon rounds, then carefully slice into bars. Serve immediately.