Place the rhubarb in a large bowl and cover with sugar. Toss to combine, then cover and refrigerate for 2 days.
After 2 days, place a fine-mesh strainer over a small bowl, and pour the macerated rhubarb mixture into the strainer, allowing the liquid to strain into the bowl. Discard the solids, or save for another use.
Pour the vinegar over the rhubarb syrup and whisk to combine. Transfer to a bottle or pint jar and store in the refrigerator.
Serve over ice, topped with sparking water.