Preheat oven to 425 degrees. Place nectarines skin-side down in a small pie plate or small baking dish. Add 1/4 cup water.

In a small bowl, stir together sugar and vanilla until completely combined. Spoon the vanilla-sugar mixture into the cavity of each nectarine.

Cover the pan tightly with foil and bake for 20 minutes. Uncover the pan and bake for about 15 minutes or more, or until the nectarines are tender and the juices are syrupy. (Add more water if the juices begin to scorch) Brush the nectarines with the syrup from the bottom of the dish. Serve warm or at room temperature.