Preheat the oven to 350°F.
In a mixing bowl, combine the mixed berries, 1/2 cup of sugar, lemon juice, and vanilla extract. Stir gently to coat the berries and set aside to marinate for about 10 minutes.
In another bowl, whisk together the flour, baking powder, salt, and remaining 1/2 cup of sugar.
Gradually add the milk and melted butter, stirring until just combined.
Pour the batter into a greased 9x13-inch baking dish.
Spoon the berry mixture evenly over the batter, but do not stir. Sprinkle cinnamon on top.
Bake for 35-40 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.
Allow to cool for at least 10 minutes before serving.