Grilled Strawberry-Rhubarb Sangria

In a small pot, combine the brown sugar and white wine with 1/2 cup of water and bring to a simmer, stirring, until the sugar dissolves. Dice 4 of the strawberries and add them to the syrup along with the chopped rhubarb and the vanilla bean. Simmer until the rhubarb is tender, 4 minutes. Transfer the syrup to a heatproof bowl and add the sprig of lemon verbena. Let cool, then refrigerate until cooled completely, about 45 minutes. Strain the syrup through a fine sieve set over a bowl, pressing on the solids. Light a grill. Thread the remaining strawberries on the skewers and grill over moderately high heat until lightly charred, about 5 minutes. Let cool completely, then cut the strawberries in half lengthwise. In a pitcher, combine the rhubarb syrup with half of the grilled strawberries and the Prosecco. Serve the sangria over ice, garnished with the remaining grilled strawberries, lemon verbena sprigs and grilled rhubarb stalks, if using.