Remove the stems and seeds from all of the dried chiles, and chop any larger ones into pieces. Next, lightly toast the chiles in a dry skillet over medium heat, until warm and slightly pliable, with small brown spots, stirring frequently, about 2-4 minutes.
Add the chiles to a microwave-safe bowl with 2 cups of the vegetable broth. Cover the bowl and microwave for 2 minutes. Add to a blender along with the fire-roasted tomatoes, and blend until smooth. Set aside.
Warm 1 tablespoon of the vegetable oil in a saucepan. Add the onion with a pinch of salt. Saute the onion, stirring occasionally, until translucent and softened, about 3-4 minutes. Add the garlic and stir, cooking for another 60 seconds or until fragrant. Finally, add the cumin and oregano, and cook, stirring constantly, for about 45 seconds or until just fragrant.
Add the blended chile paste, the potatoes, and the remaining vegetable broth to the pan. Bring the mixture to a boil and then reduce to a steady simmer. Continue cooking covered, stirring occasionally, for about 20 minutes or until hot throughout, with the flavors melded.
Meanwhile, warm the other tablespoon of vegetable oil in a skillet over medium-high heat. Add the seitan and cook, stirring frequently, for about 4-5 minutes or until slightly browned and crisped around the edges. Set aside until ready to use.
Measure out approximately 1/4 cup of the cooking liquid into a bowl. Stir it together with the masa harina or cornmeal until smooth. Add this mixture back into the simmering pot, along with the apple cider vinegar, soy sauce if using, and pan-crisped seitan, and cook for about 5-7 minutes more, or until hot throughout and thickened slightly.
Season to taste with additional salt and pepper, and serve hot.
Notes
MASA HARINA: If you can't find masa harina or don't want to buy a whole bag, cornmeal also works. If you also don't want to use cornmeal, you can make a roux with all-purpose flour at the beginning of cooking, and add that instead.