Grease a 10-inch cast iron skillet with canola oil and place it in the oven. Preheat the oven to 375 F with the skillet inside, while you work on the next steps.
Combine and mix milk and vinegar in a bowl. Add baking soda and mix well. Set aside.
Combine and mix melted butter and sugar in a bowl. Add beaten eggs and vanilla extract, mixing well. Add the milk-vinegar-baking soda mixture to the bowl, and stir until everything is combined.
Add cornmeal, flour, and salt to the bowl. Whisk until smooth without lumps. Add sliced scallions and stir them into the mixture.
Take the preheated skillet out of the oven and pour the batter onto the hot skillet.
Return the skillet to the oven and bake at 375 F until a toothpick inserted in the center comes out clean, about 25 minutes.