Grapefruit is definitely a summer fruit, and this slushy is definitely a summer drink.
Freeze grapefruit juice in an ice tray.
Create the salty simple syrup by combining the ingredients in a sauce pan over medium heat and stirring until the sugar and salt dissolve. Cool.
In a blender, add the frozen grapefruit juice, vodka, and an oz of the simple syrup and blend together.
Pour slushy into a grapefruit and salt rimmed glass.