Make the brisket: Preheat the oven to 325 degrees F. Mix 1 tablespoon chili powder, 2 teaspoons cumin, 1 teaspoon smoked paprika, 2 teaspoons salt and 1 teaspoon 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, 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 3 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 an immersion 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 in a dry skillet or in the microwave. Serve the brisket on the tortillas; drizzle with the reserved sauce and top as desired.