Pour the grapefruit juice, vodka, and triple sec into a cocktail shaker filled with ice. Shake vigorously, and strain into a cocktail glass filled with ice. Top with the lemon-lime soda and garnish with a grapefruit slice. (Alternatively, if you’d like to prepare a pitcher instead of making individual cocktails, adjust the quantities for the number of drinks you need. Mix the juice, vodka, triple sec and lemon-lime soda in a pitcher, keep cool, then pour into prepared glasses over ice when ready to serve.)