This is best eaten within a few days, but will keep in the freezer for weeks, though it may need a firm hand to break up the crystals.
Place the sugar, lime rind and juice and the lemongrass stalks in a pan with 150ml (¼ pint) water. Heat gently, stirring until the sugar dissolves then simmer gently for 5 minutes then allow to cool completely.
Strain the liquid into a rigid container then stir in the vodka and soda water. Freeze for 2 hours until almost firm then using a fork, break the mixture into large flaky crystals.