Combine the diced rhubarb and sugar in a small bowl, stirring until the rhubarb is evenly coated with sugar.
Cover the bowl and place it in the refrigerator, allowing it to rest for several days while stirring occasionally so the sugar fully dissolves and draws out the liquid.
Strain the mixture through a fine mesh strainer, pressing firmly on the solids to extract as much syrup as possible.
Mix the strained rhubarb syrup with the vinegar until fully combined and smooth.
Pour the finished shrub into a clean jar with a lid and refrigerate until ready to use.
Use immediately if desired, or let it rest for about a week so the flavors deepen and blend together more fully.
Stir into sparkling water, cocktails, or mocktails before serving cold.