Heat the oil on medium-high heat in a large iron skillet. When the oil is hot add the Brussels sprouts and stir well. Add the pepper flakes, salt, and pepper and keep stirring until the sprouts begin to crisp up and brown. Approximately 5 minutes. Reduce the heat to medium if they brown too fast. Pour the vinegar evenly over the sprouts and stir for two to three minutes or until it evaporates.