Set oven to 400F Put the flour, sugar, baking powder, baking soda, and salt in a food processor and pulse to combine. Add the butter and pulse until the mix resembles coarse crumbs. Pulse in the zucchini and walnuts, just until distributed. Mix the buttermilk with the vanilla. While pulsing, drizzle in the buttermilk. Remove the dough to a floured surface and bring together with floured hands. If it is very wet, add a touch more flour. Form into a 7-8 inch disk and cut into 8 triangles. Arrange the scones 2 inches apart on a baking sheet, and put the pan in the freezer while you clean up. Bake the scones for 20-25 minutes, until they are risen and just starting to turn golden. Cool on a rack completely before glazing. To make the glaze, whisk or stir the sugar together with enough lemon juice to make a spreadable glaze. Spread each of the scones with the glaze and top with a few chopped walnuts.