Muddle vodka, Aperol, and rosemary together in a cocktail shaker using a wooden muddler or spoon. Add ice to cocktail shaker; cover and shake until well combined and chilled.
2
Strain into 2 (10-oz. each) cocktail glasses filled with ice. Top each off cocktail with 2-oz. spiked seltzer. Garnish with rosemary sprigs and grapefruit slices, if desired.