Preheat oven to 350ºF and grease a 9-by-5-inch loaf pan. In a bowl, sift together flour, cornmeal, baking powder, baking soda and salt. Stir in poppy seeds. In a second bowl, whisk together sugar, lemon zest and juice, yogurt, oil, egg and vanilla. Stir flour mixture into yogurt mixture until just combined.
Pour batter into prepared pan and bake for 45 to 50 minutes, or until a tester inserted into center of cake comes out clean. Transfer cake to a wire rack and let cool 10 minutes, then turn out of pan onto rack and let cool completely.