Preheat oven to 350*F. Grease one 10" tube pan or bundt pan.
Sift the flour, baking soda, and salt together. Set aside. Separate the eggs.
In a large bowl, beat the egg whites until soft peaks form. Gradually add 1/2 cup of the sugar, beating until stiff glossy peaks form. Set aside.
Cream butter or margarine, 1 1/2 cups sugar, egg yolks, lemon zest, lemon juice, and vanilla together until fluffy. Add flour mixture alternately with the yogurt to the egg yolk mixture. Gently fold in the egg whites and pour the batter into the prepared pan. Bake at 350*F. for 50-60 minutes. Let cake cool in pan for 10 minutes, then turn out onto a rack to finish cooling. Enjoy!