Place the syrup, vodka and grapefruit juice into a cocktail shaker filled with ice and shake for 15 seconds. Strain the mixture over ice into two glasses. Garnish with the grapefruit zest and thyme sprigs, if desired.
To make the syrup combine all the ingredients in a small saucepan. Cook over a high heat stirring just until the sugar has dissolved. Bring to the boil then turn down the heat and leave to simmer until the rhubarb is falling apart. Push the mixture through a fine-mesh strainer placed over a bowl. Discard the pulp left in the strainer and transfer the pink syrup to a clean jar fitted with a lid. Store in the fridge until needed.