To Prepare Crust: In mixing bowl combine flour and salt; with pastry blender, or 2 knives used scissors-fashion, cut in margarine until mixture resembles coarse meal.
Add yogurt and vinegar and mix thoroughly; form dough into a ball, wrap in plastic wrap, and refrigerate for at least 1 hour (may be kept in refrigerator for up to 3 days).
Between 2 sheets of wax paper roll dough, forming a 10-inch circle about 1/8 inch thick.
Fit dough into a 9-inch pie plate and flute edges; wrap in plastic wrap and return to refrigerator.
To Prepare Filling and Bake: Preheat oven to 450°F.
In large mixing bowl, using electric mixer, cream margarine with sugar until light and fluffy; beat in egg yolks.
Add flour, orange peel, juice, and vanilla and beat until well blended; stir in buttermilk.
In small mixing bowl, using clean beaters, beat egg whites until stiff but not dry; fold into yolk mixture.
Pour filling into prepared pie crust and bake for 15 minutes.
Reduce oven temperature to 350°F and continue baking for 45 to 50 minutes longer.
Remove pie plate to wire rack and let cool; cover and refrigerate for at least 1 hour before serving.