3 nectarines (1 1/4 pounds), pitted and cut into 1/2-inch pieces
In a small bowl, combine the ice water, egg yolk and vinegar. In a food processor, pulse the 3/4 cup plus 2 tablespoons of flour with the cornstarch and salt. Add the butter and pulse until it is the size of peas. Sprinkle on the egg mixture and pulse just until a soft dough forms. Turn the pastry out onto a floured work surface and roll it into an 8-inch log. Wrap in plastic and refrigerate until firm, about 30 minutes.
Preheat the oven to 375°. Line 2 baking sheets with parchment paper. In a saucepan, bring the sugar, water and vanilla bean and seeds to a boil, stirring until the sugar dissolves. Remove from the heat. Add the lemon juice and nectarines; let cool.
On a lightly floured work surface, cut the pastry log into sixteen 1/2-inch slices. Using a floured rolling pin, roll out each slice to a 3-inch round and transfer to the baking sheets. Drain the nectarines and arrange on the pastry rounds, leaving a border all around. Fold the border up and over the nectarines, pleating the edges; the tartlets should be about 2 inches wide.
Bake the tartlets for about 25 minutes, until they are golden and bubbling; shift the pans halfway through baking. Let cool and serve.