Preheat the oven to 350 degrees F. For the filling, in a large mixing bowl, combine the blueberries, nectarines, granulated sugar, lemon zest and cornstarch. Place the fruit mixture in a wide shallow baking dish (2 to 2 1/2 quarts) or eight 1-cup ramekins. Set aside.
For the topping, in a food processor, combine the flour, oats, brown sugar, cinnamon and a pinch of salt and pulse.
Add the butter and vanilla to the food processor and pulse, pulse, pulse, until the mixture looks dry and crumbly. Add 1 tablespoon water and pulse again until the mixture starts to come together. If the mixture still seems dry, add 1 more tablespoon water and pulse again. Sprinkle the topping over the blueberry mixture.
Bake the crisp until the filling is hot and bubbly and the topping is crispy and nicely browned, 35 to 40 minutes. Serve warm with ice cream or whipped cream.