In a large, heavy Dutch oven, heat oil over medium-high heat. Add onions, bell pepper, and celery and cook, stirring frequently, until vegetables have softened, 5 to 7 minutes. Add thyme, bay leaves, 1/2 teaspoon cayenne, 2 teaspoons salt, and garlic and cook, stirring, until garlic is fragrant, 1 minute. Add tomato puree, cook 2 minutes longer, then add stock and crabs (if using), and bring to a boil. Reduce to a simmer and cook for 45 minutes.
Add okra and vinegar, and bring to a boil. Skim any foam that comes to the surface, reduce heat to a simmer, and cook, partially covered, until okra has fallen apart to create a thickened sauce, about 2 hours. (You will see that the longer the okra cooks, the less "slimy" it will be.) The sauce should be thick and rich; should it get too thick, thin with a bit of water.
Season shrimp with the remaining 1/2 teaspoon salt and 1/4 teaspoon cayenne and stir well. Stir the shrimp into the hot gumbo and cook until just cooked through, about 3 to 5 minutes. Remove bay leaves and thyme sprigs; discard. Serve gumbo in shallow bowls, ladled over cooked white rice, with hot sauce, parsley, and scallions at the table for guests to add to their liking.
Note: Gumbo crabs are blue crabs that are not graded #1, i.e., they may not be as heavy or full of crabmeat and thus they are kept aside and sold as gumbo crabs for folks to use to flavor soups and stews. You can often find them in the freezer section in grocery stores in Louisiana, but if you cannot find them where you live, you can simply substitute 2 regular blue crabs if desired. Gumbo crabs are used mostly to flavor the gumbo and are not typically eaten at the table.
Yield: Makes 3 1/2 quarts