Heat oven to 425°. In a small bowl, blend together 3/4 cup of the yogurt, buttermilk, egg, oil and lemon zest. Stir in diced peaches.
In a large bowl, mix flour, 1/3 cup of the sugar, baking powder, baking soda and salt. Make a well in flour mixture; add yogurt mixture to well. Gently stir just until evenly moistened and dough comes together.
Place dough on a lightly floured work surface and with floured hands shape into a 14-inch log. With a floured knife, cut into 10 equal pieces, each about 1 1/2-inches thick. Shape each piece into a 4-inch round. Transfer to baking sheet. Brush rounds with a little water, then sprinkle with 1 tablespoon of the remaining sugar.
Bake for about 18 minutes, or until golden brown and toothpick inserted in center comes out clean. Remove to a wire rack and cool completely.
Meanwhile, whip heavy cream with remaining 1 tablespoon sugar to medium-stiff peaks. Fold in remaining 1/2 cup vanilla yogurt.
Cut each shortcake horizontally in half. Place each bottom half on a platter or plate. Spoon a little juice from the peaches onto each, then divide peaches and whipped cream among shortcakes. Cover with top half of shortcake, cut side down and serve.