In a large bowl, whisk together flour and salt. Cut in cold butter using a pastry blender until mixture resembles coarse crumbs.
Gradually add ice water, mixing until dough just comes together. Form into a disc, wrap in plastic, and chill for 30 minutes.
Filling Preparation
In a saucepan, combine brown sugar, corn syrup, melted butter, vinegar, and vanilla extract. Bring to a simmer over medium heat, stirring until sugar is dissolved.
Assembly and Baking
Preheat oven to 375°F (190°C). Roll out dough on a lightly floured surface and press into a 9x13 inch baking pan.
Sprinkle raisins (if using) over the crust. Pour filling over the raisins and crust.
Bake for 30-35 minutes, or until crust is golden brown and filling is set. Let cool completely before cutting into squares.
Notes
For a richer flavor, try using dark brown sugar. Store leftovers in an airtight container at room temperature.