In a liquid measuring cup, measure out ¾ cup yogurt and squeeze in about ¼ cup citrus juice to yield 1 cup total liquid. Whisk the yogurt and juice, eggs and vanilla into the sugar mixture until well blended.
Pour the sugar into another medium-sized mixing bowl. Use a Microplane to grate 2 teaspoons zest from your fresh citrus fruits and add it to the bowl. Rub the zest into the sugar until the sugar takes on the color and fragrance of the citrus.
Preheat the oven to 350 degrees Fahrenheit. Generously butter and flour an 8½ by 4¼-inch loaf pan. In a medium mixing bowl, whisk together the flour, baking powder and salt.