Start by finely dicing the apples. To get 1 ¼ cups, I use anywhere from 1-1 ½ apples. Then, set the diced apples aside.
Add the all-purpose flour, sugar, baking powder, apple pie spice, and salt to a large mixing bowl. Whisk together.
Cut the cold vegan butter into chunks. Then, add it to the bowl of dry ingredients and use a pastry cutter to "cut" the butter into the dough until you have a crumbly mixture. You can also use a fork to do this step, though it will take longer.
Next, add in the oat milk and vanilla. Use the pastry cutter or your hands to combine into a shaggy dough. The shaggy dough will be somewhat flaky and crumbly, yet still stick together when pressed.
As a final step, add in the diced apples and use a spoon to mix them into the dough.