In a large cast iron pot, heat oil over medium-high heat. Add pork shank and vegetables to oil. Cook 2-3 minutes, stirring frequently until vegetables begin to soften.
Add all of the seasonings to the pot and stir well. Stir in water and chicken base. Allow mixture to boil for 5 minutes and add drained cowpeas. Lower heat, cover and cook for 40-45 minutes until peas are tender.
Taste broth and add additional seasonings if desired. Stir in rice and cover.
Let simmer 20-30 minutes until rice is tender and liquid has fully absorbed into the rice and peas. Remove pot from heat. Stir in a splash of rice vinegar. Spoon rice and peas into bowl or plate. If desired, garnish with chopped scallions or chives.