Combine the raspberries, sugar and ¾ cup water in a small saucepan. Bring to a boil. Reduce the heat and simmer, uncovered, until the raspberries begin to fall apart and the sugar dissolves, about 5 minutes.
Strain the liquid through a wire-mesh strainer into a glass measuring cup, pressing the solids to release the juices. Discard the solids and let the simple syrup cool completely.
To serve, pour 2 tablespoons simple syrup in the bottom of each of 8 champagne flutes.
Top with ⅓ cup grapefruit juice and about ⅓ cup sparkling rose.
Top each with about 1 tablespoon vodka and garnish with raspberries and grapefruit slices.
