Preheat oven to 350 F. In a medium bowl, whisk together flour, cornmeal, sugar, salt and baking powder. In another bowl, whisk together milk and vinegar. Pour the milk mixture, eggs and vegetable oil into the dry ingredients, whisk until smooth.
Pour the cornbread mixture into a large greased baking dish. Bake for 30 minutes. Let cool. Crumble cornbread into a large greased crock-pot. In a small saucepan over medium heat, melt butter.
Add in diced onion and diced celery. Cook until softened. Mix cooked vegetables with cornbread and pour in vegetable broth and season with poultry seasoning. Combine all ingredients together in crock-pot. Cover and let cook on low for 4-5 hours, or on high for 2-3 hours.