Wipe okra gently with moist kitchen towel. Cut each into 4 pieces and set aside. Heat vegetable oil in a skillet. Add onions, garlic, cumin seed, chilies (optional), and coriander leaves. Saute 4 to 5 mins. Add turmeric. Stir once or twice. Add tomatoes, vinegar and half cup water. Bring to a boil. Lower heat and simmer 15 mins. Add okra, black pepper and salt. Cover and cook until okra is tender, about 20 mins.