Prepare the pear filling by peeling and dicing the pears, then toss with lemon juice, granulated sugar, cornstarch, and vanilla extract.
Make the crumble topping by mixing rolled oats, flour, brown sugar, salt, and cinnamon, then add melted butter and mix until crumbly.
Assemble the bars by pressing two-thirds of the crumble mixture into a greased baking dish, spreading the pear filling on top, and sprinkling the remaining crumble over the pears.
Bake in a preheated oven at 350°F for 30-35 minutes until golden brown and bubbly.
Allow to cool for at least 15 minutes before slicing into bars.