Place ribs, chorizo, onion and garlic in bottom of slow cooker. Place collard greens on top, pressing down firmly. Whisk stock, salt and pepper, then pour on top. Cover and cook on HIGH for 5 hours or LOW for 7 hours.
Carefully remove ribs to a cutting board and shred; discard bones. Stir beans and vinegar into liquid. Stir meat back into slow cooker.
To serve, scatter about 1/2 cup rice into each bowl. Ladle some stew on top and garnish with orange segments.