Cover black-eyed peas with water in a large soup pot, bring to a boil, and boil for 2 minutes. Reduce heat to low and simmer peas for 1 hour. Drain.
Dissolve vegetable bouillon cube in 1/4 cup hot water in a small bowl; pour bouillon into slow cooker. Stir black-eyed peas, onion, garlic, ham hock, collard greens, oregano, red pepper flakes, and vegetable broth into the cooker. Add 5 cups of water or as needed to cover. Cook 5 hours on High. Stir rice into stew and cook 1 more hour.
Remove meat from ham hock, chop meat, and return to stew. Season with salt and black pepper.
