Preheat your oven to 350°F (175°C). Grease a 9-inch square baking dish.
In a large bowl, combine the diced mangoes, lemon juice, granulated sugar, and vanilla extract. Toss gently to coat the mangoes evenly, then spread the mixture evenly in the prepared baking dish.
In another bowl, mix the rolled oats, all-purpose flour, brown sugar, cinnamon, and salt. Pour in the melted butter and stir until the mixture is crumbly and well combined.
Sprinkle the oat mixture evenly over the mango layer in the baking dish.
Bake in the preheated oven for 30-35 minutes, or until the topping is golden brown and the mangoes are bubbly.
Remove from the oven and let it cool for about 10 minutes before serving.