Mix goat meat with vinegar, soy sauce and garlic in a large bowl; cover and refrigerate for 1 to 8 hours. For best flavour, marinate at least 6 hours. Remove meat from marinade and pat dry with kitchen paper; reserve marinade and garlic cloves.
Heat vegetable oil in a large pot over medium-high heat and brown the goat meat, working in batches if necessary, 10 to 15 minutes. Set goat meat aside. Cook and stir onion, red pepper and garlic cloves from the marinade over medium heat until onion has softened, about 5 minutes; pour in tomato passata and bring mixture up to a simmer. Allow to cook down slightly, about 5 minutes.
Return goat meat to the sauce and pour in reserved marinade and beef stock. Bring to the boil; reduce heat to low and simmer covered until goat meat is partially tender, 30 to 40 minutes. Stir in potato, carrots and peas; season with salt, black pepper and cayenne pepper. Simmer until goat meat is very tender and potato and carrots are cooked through, 20 to 30 more minutes. Adjust seasonings before serving.