Combine the rhubarb, water, and sugar in a small saucepan over medium-high heat. Bring to a boil, then reduce the heat to low and simmer for 20 minutes. Pour the syrup through a fine mesh strainer and press against the solids to extract as much flavor as possible. Let the syrup cool completely.
Combine the vodka, grapefruit juice, and rhubarb syrup in a cocktail shaker filled with ice and shake for about 30 seconds, until cold. Strain into a chilled glass, top off with club soda, and serve.