For the rhubarb syrup, add the sugar, water, vanilla, and chopped rhubarb to a medium saucepan. Bring to a boil over medium heat, and let cook for 3 minutes, or until rhubarb is tender.
Strain the mixture through a fine cloth or sieve to remove pulp.
Pour the syrup into a container and store for up to 1 week in the refrigerator.
To make the cocktail, put the gin, 9 ounces of rhubarb syrup, and the lemon juice in a pitcher with ice. Stir to combine.
Pour into glasses filled with ice, top with prosecco, and garnish with a lemon wheel. Cheers!