Lime ice cubes
Place the slices of lime in an ice cube tray, fill with water, freeze for approx. 2 hrs.
To reduce the rhubarb mixture
Place the rhubarb, lime juice, vanilla pod, vanilla seeds and sugar in a pan, bring to the boil while stirring, reduce over a medium heat for approx. 20 mins. Remove the vanilla pod, puree the rhubarb, strain.
Bring the rhubarb liquid, water and sugar to the boil while stirring. Simmer over a low heat for approx. 15 mins. Remove the pan from the heat, leave to cool.
Pour 1/2 tbsp of syrup into each glass, fill with Prosecco. Add the lime ice cubes.