In a large bowl, stir together onions, okra, bell pepper, celery, 1/4 cup parsley, garlic, bay leaves, salt, thyme, cayenne, black pepper and allspice. Set aside.
In a large Dutch oven, cook chicken in batches over medium heat until browned, about 5 minutes a side; transfer to a large bowl. Cook sausage in Dutch oven over medium heat, stirring constantly until lightly browned, about 6 minutes; transfer to bowl with chicken. Pour fat from Dutch oven into a glass measure and add enough vegetable oil to equal 2/3 cup. Pour into Dutch oven and heat over medium-low heat, scraping up any browned bits. Gradually whisk in flour and cook, stirring frequently, until it is a very dark, rich brown, about 30 minutes; be careful not to burn it.
Add broth all at once and scrape up any browned bits. In another bowl, blend tomato paste with 2 cups water and stir into broth with chopped tomatoes. Add reserved vegetables, chicken and sausage. Bring to a boil over high heat; reduce heat to low and simmer for 1 hour, until thickened.
Stir in remaining 1/2 cup parsley, scallion greens and vinegar and cook, stirring, for 5 minutes. Remove Dutch oven from heat, cover and let stand for 10 minutes. Discard bay leaves.
Serve gumbo on top of rice in bowls.