In a shaker with ice, combine vodka, lemon juice, and keto simple syrup. Shake until very cold.
Sugar the rim of a cocktail glass if you prefer.
Strain and pour into cocktail glass. Top with soda water and garnish with slice of lemon.
Notes
Sugar free simple syrup: Mix 1 cup of water with 1 tablespoon of monk fruit liquid sweetener. This can be used as a 1:1 replacement for simple syrup. Store in the fridge up to a month.