In a medium saucepan, add the blueberries and lemon juice. Cook over medium heat until blueberries are tender, about 8 minutes. In a small bowl, whisk together the sugar and cornstarch. Add the sugar mixture to the pan and stir to combine. Bring to a boil and cook for 1 to 2 minutes, until thickened. Transfer filling to a medium bowl and set aside to cool slightly.

Preheat the oven to 350 degrees F. Lightly grease a 9- X 13-inch baking pan.

In a food processor fitted with a knife blade, add the flour, oats, brown sugar, baking soda, and salt. Pulse one or two times to combine. Add the diced butter and vanilla. Pulse until it resembles coarse crumbs. Set aside 1 1/2 cups of the crumb mixture in a bowl to be used for the topping.

Pour the remaining crumb mixture into the baking pan, spread evenly, and pat down. Place filling in small dollops over the crust. Carefully spread the filling evenly over the crust. Sprinkle reserved crumb mixture over the filling.

Bake until golden brown, about 30 to 35 minutes. Cool completely on a wire rack. Slice into 20 bars. Serve at room temperature. Store in an air-tight container for up to 3 days.