In a large mixing bowl, whisk together the flour and salt. Cut in the cold butter until the mixture resembles coarse crumbs. In a separate bowl, mix the egg, vinegar, and cold water. Gradually stir this mixture into the flour until just combined. Knead gently, wrap in plastic, and refrigerate for 30 minutes.

Heat olive oil in a skillet over medium heat. Sauté the chopped onion and red bell pepper until softened, about 5 minutes. Add minced garlic, cumin, smoked paprika, and oregano; cook for 1 more minute. Stir in shredded chicken and tomato sauce, seasoning with salt and pepper. Allow the filling to cool completely.

Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper. Roll out the chilled dough to about ⅛ inch thick. Cut circles, approximately 4 inches in diameter. Place a spoonful of filling onto each circle, then fold the dough over to create a half-moon shape. Seal edges using a fork.

Transfer the filled empanadas to the baking sheet. Beat the remaining egg and brush it over the tops. Bake for 25 to 30 minutes, or until golden brown and the filling bubbles. Let cool slightly before serving.