Coat the inside of a 4- to 5-quart oval slow cooker or a 4-quart round slow cooker with cooking spray. In a large bowl stir together flour, poppy seeds, baking powder, and salt. Set aside.
In a medium bowl whisk together sugar, eggs, oil, yogurt, milk, lemon peel, lemon juice, and vanilla until sugar dissolves. Add sugar mixture all at once to flour mixture. Stir just until combined (mixture should still be slightly lumpy). Spoon batter into prepared slow cooker.
Cover and cook on high-heat setting for 1-1/2 to 2 hours or until top appears set. Turn off slow cooker. Carefully remove lid so condensation from lid does not drip onto bread. Cover opening of slow cooker completely with paper towels; place lid on top. Cool for 10 to 15 minutes. Run a knife around edges of cooker; remove bread from cooker. Cool completely on a wire rack.