|Bring sugar and water to a boil in a small saucepan, stirring just until sugar dissolves; remove from heat. Cool.
Place coarsely chopped rhubarb in a wide-mouth jar. Add vodka, Grand Marnier, and cooled sugar syrup; stir.
Screw lid on tightly; let stand at room temperature for 2 to 3 weeks or until all the color leaches out of rhubarb. Strain mixture through a sieve over a bowl; discard solids.
Serve with seltzer or straight up and ice cold.