Bring 1 cup water and confectioners' sugar to a boil in a saucepan; reduce heat to medium-low and simmer until a syrupy consistency is reached, about 5 minutes. Remove from heat and cool to room temperature.
Combine syrup, grapefruit juice, 1 cup water, and vodka together in a bowl; transfer to the bowl of an ice cream maker. Freeze for 3 to 4 hours.
Process mixture in the ice cream maker according to manufacturer's instructions for sorbet. Freeze for an additional 4 to 6 hours.