Spread peanut butter over four slices of bread; top with banana slices. Place remaining bread on top. Cut each sandwich in half; arrange in a greased 8-in. round baking pan. Whisk together eggs, milk and vanilla; pour over sandwiches. Refrigerate, covered, overnight.
Preheat oven to 350°. Remove French toast from refrigerator while oven heats. Bake, uncovered, until golden brown, 25-30 minutes. Serve with syrup.