In a mixing bowl, combine the sliced peaches, 1/2 cup of sugar, lemon juice, and vanilla extract. Toss until the peaches are well-coated. Set aside and let them marinate for 10 minutes.

In a large bowl, stir together flour, oats, brown sugar, 1/2 cup of sugar, and cinnamon. Pour the melted unsalted butter over the mixture and combine until crumbly and well-mixed.

Preheat your oven to 350°F (175°C). Press half of the crumble mixture into the bottom of a greased 9×13 inch baking dish to form a base.

Spread the marinated peaches evenly over the crust, then sprinkle the remaining crumble mixture over the top.

Place your baking dish in the oven and bake for about 30 minutes, or until the top is golden brown and the peach filling is bubbly.

Remove the bars from the oven and let them cool for at least 15 minutes before slicing. Serve warm or at room temperature, with optional whipped cream or ice cream.