Blend together in a food processor half of the black beans and the canned tomatoes. Process until nearly smooth. Set aside.
Heat the olive oil in a large soup or stock pot to medium-high heat. Add the diced onions, poblano, garlic, pancetta (if using, salt, and pepper. Stir together, and allow the vegetables to thoroughly soften as the pancetta crisps up a bit, stirring frequently and adjusting heat to prevent them from browning, about 7 minutes. Add the cumin and Mexican oregano, stir, and allow to cook another 3 minutes or so.
Add the blended bean/tomato mixture to the sauteed vegetables and stir. Stir in the pumpkin, and then stir in the water and stout (if using). Simmer and stir frequently for 20 minutes or so to slightly condense and thicken, and to let the flavors come together. Stir in the remaining whole beans and serve. If you are making this in advance of serving time, add the beans in the last five minutes before serving to they don't overcook and collapse.
Right before serving, stir in the vinegar and taste to adjust seasoning. Ladle into bowls, serve plain, or garnish with your favorite combination of topping ingredients.