Cut the plums into quarters or smaller, if they are large. Remove the pit and discard. You can leave the skins on.
Preheat oven to 350°F and spray a 9x13-inch baking dish with nonstick cooking spray or grease it with butter.
Combine the flour, oats, sugar, brown sugar, vanilla, cinnamon, baking powder, and salt in a large bowl.
Add the butter and mix with a pastry blender, fork, or your fingers until combined. Mix until crumbly but still holding together when pressed together.
Set aside 1 cup of crust mixture.
Transfer the remaining crust mixture to the prepared baking pan. Press firmly and evenly into the bottom of the pan - a spatula or offset spatula works great for this.
Bake the crust for 10 minutes in the preheated oven. Remove from oven.
Layer the plums over the crust, overlapping each slice a bit. Sprinkle the reserved crust mixture over the plums.
Bake for 35 to 40 minutes or until the crust is golden brown. Cool completely before cutting into bars, then enjoy!