Stir sugar and 1/2 cup water together in a saucepan over medium heat until sugar has dissolved and mixture comes to a boil. Remove from the heat and let cool completely, 15 to 30 minutes.
Pour cooled syrup into a large mixing bowl. Add shortening, oil, and vanilla; beat with an electric mixer on high speed until well blended. Beat in confectioners' sugar, 1 cup at a time, until filling is thick and fluffy. Mix in water, 1 tablespoon at a time, if filling is too thick.