In a medium mixing bowl, combine the graham cracker crumbs, flour, brown sugar, and salt.
In a small mixing bowl, whisk together the melted butter and vanilla.
Using a spoon, mix and stir the melted butter into the graham cracker mixture until the crumbs are moist.
Spread crumb topping over the fruit, pressing down with the back of a spoon or with your hands to create an even layer.
Bake for 25 minutes, or until the crust is golden brown and mixture is bubbling up.
Remove apple crumble from oven, then set oven to BROIL.
In the meantime, top the apple crumble with marshmallows in one single layer.
BROIL the apple crumble for 3 to 4 minutes, or until marshmallows are browned and toasted on top. Watch it closely so that the marshmallows don't burn.
Remove from the oven and let cool for 5 to 7 minutes.
Serve warm or at room temperature.