Preheat oven to 350°F. Line an 8x8 pan with foil or parchment paper (leaving overhang on two sides for easy removal). Spray with nonstick spray.
In a medium bowl, combine, flour, oats, salt and baking soda.
In a large bowl, whisk butter, brown sugar and vanilla until well combined (this will take at least one full minute). Add dry ingredients and stir with a rubber spatula or wooden spoon until combined.
Pull out about ¾ cup (a little less than half) of the dough and set that portion aside (it will be the topping). Dump the remaining dough into the prepared pan and press it down into a flat even layer (it will be very buttery, that's ok). Bake for 10 minutes.
Meanwhile, whisk all filling ingredients until combined. After the crust has baked for 10 minutes, pour the filling over the crust. Gently spread into an even layer. Be careful, the crust underneath will be soft and undercooked, you don't want to break it. Crumble the reserved dough over the filling and return to the oven for 17-20 minutes, until the top is light golden brown and the filling is set. Place the pan on a wire rack to cool completely.