Heat the broiler to high with the oven rack 6-inches from the heating element. Wrap a sheet pan or large jelly roll pan in aluminum foil for easy clean-up, if desired.
Use a pastry brush to "paint" the melted butter across the pan covering all of the surface areas of the pan.
Beat the eggs, milk, lemon zest, and vanilla together in a shallow dish. One by one, dip the bread slices into the egg mixture until the centers of the bread slices are soaked through. Allow any excess egg mixture to drip off each slice before placing the slice on the prepared pan.
Repeat the process with all of the slices of bread, nestling the slices close to each other. Add the bunches of grapes to the pan next to the soaked bricohe.
Broil until the bread is toasted and starting to turn golden brown, about 4 minutes. Flip the bread slices and place them back in the oven for another 3-4 minutes until completely toasted and the grapes are starting to burst.