1. Preheat oven to 375 degrees F.
2. In large bowl, combine nectarine slices, blueberries, lemon juice, granulated sugar, cornstarch, cinnamon, and vanilla.
3. In separate bowl, mix together flour, oats, dark brown sugar, and salt. Add butter and, using clean hands, combine butter with dry ingredients until small clumps form.
4. Pour fruit mixture into 9x13-inch baking pan and sprinkle topping evenly over fruit mixture. Bake in oven for approximately 45 minutes, until fruit is cooked through and bubbling and topping is light brown and crisp. Let cool for about 10-15 minutes before serving warm.