Heat the sugar and water in a small saucepan over medimedium-hight until the sugar is fully dissolved.
Add the lemon and passionfruit and bring to a boil. As soon as the syrup is boiling, remove from heat and let cool.
Make the Passionfruit Mocktail or Cocktail:
Fill two cocktail glasses with ice. Cover the bottom of each with about 2 tablespoons of passion fruit simple syrup. Fill to 2/3 full with soda water and also vodka if using. Stir and top with remaining passion fruit simple syrup.