Preheat your oven to 350°F (175°C) and grease three 9-inch round cake pans. In a large bowl, mix together the flour, canola oil, white sugar, buttermilk, eggs, vinegar, baking soda, 1 teaspoon vanilla extract, and cocoa powder until well combined, about 2 to 5 minutes. Pour the batter evenly into the prepared pans. Bake in the preheated oven until the cakes turn golden brown, approximately 20 minutes. Let the cakes cool in the pans for about 30 minutes. Meanwhile, in a saucepan over medium heat, combine the sweetened condensed milk, dark brown sugar, butter, 2 teaspoons vanilla extract, and salt. Stir constantly and bring to a boil, cooking until the mixture thickens, about 3 to 5 minutes. Allow the frosting to cool for about 5 minutes. Once the cakes have cooled completely, remove them from the pans and spread the caramel frosting evenly between the layers. Stack the layers, then frost the top and sides of the cake.