Preheat oven to 350°F (175°C). Line an 8x8-inch baking dish with parchment paper.
In a large bowl, mix oats, flour, brown sugar, salt, and cinnamon. Add melted butter and vanilla. Stir until crumbly.
Press about 2/3 of the mixture into the bottom of the prepared pan to form the crust.
In another bowl, mix mangoes, sugar, lemon juice, and cornstarch. Spread this filling evenly over the crust.
Sprinkle the remaining oat mixture on top as the crumble layer.
Bake for 30–35 minutes, or until the top is golden and the mango filling is bubbly.
Let cool completely before slicing into bars.