Preheat oven to 180°C (350°F).
In a large bowl, toss berries with granulated sugar, lemon juice, cornstarch, and vanilla extract until evenly coated. Transfer to a baking dish and spread evenly.
In another bowl, combine flour, oats, brown sugar, cinnamon, and salt. Add cold butter cubes and work into the mixture using fingers or a fork until crumbly.
Sprinkle topping evenly over berries, leaving some gaps for steam to escape. Do not pack down.
Bake for 35-40 minutes, until topping is golden brown and berries bubble around edges.
Let rest for at least 10 minutes before serving. Serve warm.