In a small bowl, stir together flour, poppyseeds, salt and baking soda. In large bowl, cream together sugar and butter. Beat in eggs, one at a time. Beat in yogurt and vanilla until well blended. Stir in flour mixture until moistened toroughly. Spoon batter into greased muffin tins and bake at 400 deg. F for 15-20 minutes or until a wooden pick inserted in center comes out clean.