Heat broiler to high with the rack 6 inches from the flame. Lightly grease a baking sheet with butter.
Beat the eggs, milk, and vanilla together in a shallow bowl. Dip the bread slices in the egg mixture, in batches, until the centers are soaked through. Let any excess egg drip off.
Place the bread slices on the greased pan. Lay the bunches of grapes alongside them. Broil until the bread is toasted and golden, about 3 minutes. Meanwhile, stir together the sugar and cinnamon. Flip the bread and broil on the other side until lightly toasted, about 2 minutes. Remove the bread and sprinkle with the cinnamon sugar. Allow the grapes to cook for another 5 minutes.
Serve warm with more butter and maple syrup.