Preheat the oven to 350 degrees F and line an 8-inch square baking pan with parchment paper.
Whisk the flour, oats, sugar, baking powder, and salt together. Stir in the melted vegan butter and vanilla until large, moist crumbs form.
Reserve 1 cup of the crumble mixture. Firmly press the remainder into the prepared pan and bake for 12 minutes.
Toss the blueberries with the sugar, cornstarch, lemon zest, lemon juice, and vanilla until evenly coated.
Spread the blueberry filling over the warm crust. Scatter the reserved crumble evenly across the berries.
Bake for 38 to 43 minutes, until the filling is bubbling around the edges and the crumble is golden.
Cool the bars completely in the pan, then refrigerate for 1 hour before portioning so the blueberry layer sets cleanly.
