Arrange racks in upper and lower thirds of oven; preheat to 425°F Line 2 baking sheets with parchment paper.
Whisk 3 cups flour and next 5 ingredients in a large bowl. Add butter; rub in with your fingers until mixture resembles coarse meal.
Whisk 1 cup buttermilk, zest, and vanilla in a small bowl. Add wet ingredients to dry ingredients. Stir until shaggy dough forms.
Transfer to a lightly floured surface; knead until dough forms, about 5 turns. Pat into a 10x6" rectangle. Halve dough lengthwise. Cut each half crosswise into 4 squares. Cut each square diagonally in half into 2 triangles. Divide between baking sheets. Brush scones with 2 tablespoons buttermilk. Sprinkle with sanding sugar.
Bake until scones are golden and a tester inserted into the center comes out clean, 13-15 minutes. Transfer to wire racks; let cool.
Serve warm or at room temperature with lemon curd.