Preheat oven to 350 F. Butter 13x9 inch baking dish. In large bowl, whisk sugar, flour, zest and apple pie spice. Add apple slices and vanilla; toss to coat. Spoon filling into prepared dish. Sprinkle topping over. Bake until apples are tender and topping is golden and crisp, about 1 hour. Cool at least 10 minutes. Serve warm with ice cream if desired.