Add rhubarb and sugar to a small saucepan, and bring to a gentle simmer over medium-low heat.
Continue to cook, stirring occasionally, until the sugar is dissolved and the rhubarb has softened, around 5 to 8 minutes.
Remove from heat and let cool for 15 minutes.
Pour the rhubarb syrup through a fine-mesh sieve into a large measuring cup or bowl, then transfer to a bottle or Mason jar with a tight-fitting lid.
Add vinegar to the rhubarb syrup and shake the bottle to combine, or mix well with a spoon.
Seal and refrigerate for up to 3 months.
To serve, mix one part shrub with four to five parts sparkling water, juice or another mixer of your choice.