Make the filling: In a large bowl, whisk together the sugar, flour, salt, lemon juice and zest, and the vanilla bean seeds.
Add the blueberries and peaches and use a spatula to toss until well coated.
Transfer the filling to a 10-inch (25 cm) cast-iron skillet. Refrigerate until ready to top.
Make the topping: In a large bowl, whisk together the flour, oats, sugar, salt, cinnamon, and nutmeg.
Add the butter and quickly work into the dry ingredients until the butter is broken down into pea-sized pieces.
Sprinkle the topping in an even layer over the filling.
Preheat the grill to 350°F (180°C).
Place the cast-iron skillet on the grill and shut the lid. Grill the crumble for about 1½ hours, until the topping is golden brown and the fruit it bubbling. Check after about 1 hour to make sure it is not overcooking. Let cool for 30 minutes before serving.