How to Make It
Pour grapefruit juice in ice cube trays, and freeze overnight or until solid.
Bring sugar, water, and kosher salt to a boil in a small saucepan over high. Reduce heat to medium; cook, stirring constantly until sugar and salt dissolve. Remove from heat, and cool to room temperature, about 30 minutes. Syrup may be covered and refrigerated for up to 2 weeks.
To make the slushies, in batches, combine half of the frozen juice cubes, half of the vodka, and 1/4 cup of the salted simple syrup in a blender or food processor, and process until smooth, but still frozen. Repeat with remaining ingredients.
Dip the rims of serving glasses in grapefruit juice, then in salt to coat the rims. Spoon 1 cup of the slushy mixture into each glass and enjoy immediately.