Make the brisket: Preheat the oven to 325 degrees F. Mix 1 tablespoon chili powder, 2 teaspoons cumin, 1 teaspoon smoked paprika, 1 teaspoon onion powder, 1 teaspoon garlic powder, salt and pepper in a small bowl. Score the fat on the brisket with the tip of a sharp knife (do not cut into the meat), then rub the spice mixture all over the brisket until coated. Heat the vegetable oil in a large Dutch oven over medium-high heat. Add the brisket fat-side down and cook until browned on both sides, 5 to 6 minutes. Transfer to a plate.

Add the poblanos, red onion, garlic, the remaining 1 tablespoon chili powder, 2 teaspoons cumin and 1 teaspoon smoked paprika, 1 teaspoon onion powder, 1 teaspoon garlic powder, the coriander, oregano and a pinch of salt to the pot. Cook, stirring frequently, until the vegetables soften, 3 to 5 minutes. Stir in the tomato juice and chicken broth and remove from the heat. Nestle the brisket fat-side up in the liquid and cover the pot. Bake until the meat is very tender and a thin knife inserted into the center twists easily, about 4 hours.

Transfer the brisket to a cutting board. Spoon off and discard the excess fat from the sauce in the pot, then puree until smooth using a stick blender (or transfer to a regular blender and puree). Stir in the vinegar and season with salt. Transfer about 3/4 cup of the sauce to a small bowl for serving. Stir 2 tablespoons water into the remaining sauce in the pot. Thinly slice the meat against the grain and return to the pot. Cook over medium heat, stirring occasionally, until the meat is well coated with sauce and warmed through.

Heat the tortillas over a gas flame until lightly charred. Serve the brisket on the tortillas and drizzle with the reserved sauce and top with your favorite toppings.