CARAMEL TOPPING: In a small pot, cook 1/4 cup brown sugar, condensed milk, 4 tablespoons of butter, and salt over medium-low heat. Stir gently for about 5 minutes, then add the vanilla extract. Stir for another minute, then remove from heat.
In large bowl, stir together caramel topping and the cinnamon until blended. Add apples; toss until evenly coated. Spread mixture evenly in an ungreased 8-inch square glass or ceramic baking dish.
In same bowl, mix flour and remaining brown sugar. Cut in butter, cutting 2 table knives through mixture in opposite directions, until mixture looks like coarse crumbs. Stir in oats. Crumble mixture evenly over the caramel apples in the baking dish.
Bake 45 to 50 minutes or until apples are tender and topping is golden brown.