Bring 1 cup of water to a boil. In a dry cast-iron skillet, toast the chiles over medium-low heat until fragrant, turning them and shaking the pan so they don't scorch, about 2 minutes. Put the toasted chiles in a bowl, cover with the boiling water and let them soak until softened and reconstituted, about 15 minutes.
Using a sharp knife, make a deep slit down the length of the tenderloin and open it up so it lays flat. With the smooth side of a mallet, pound the pork between 2 pieces of plastic to a 1/4- to 1/8-inch thickness. Transfer to a baking dish.
In a blender, combine the guajillo chiles, olive oil, vinegar, oregano, 1 teaspoon salt, 1/2 teaspoon pepper and 2 tablespoons of the reconstituting liquid and puree to form a paste. Rub the paste on both sides of the pork. Cover with plastic wrap and refrigerate overnight.
Preheat an outdoor grill or a griddle pan over a medium-high flame. Drizzle the grill with a little vegetable oil to prevent the meat from sticking. Re-season the pork with a sprinkle of salt and pepper and grill for 2 minutes per side. Remove to a cutting board and let rest for 2 minutes to allow the juices to settle. Dice into 1/4-inch pieces.
Heat a large dry skillet over a medium flame. Warm the tortillas until toasty and pliable, about 30 seconds on each side.
To make the tacos, lay about 4 ounces of the pork down the center of 1 tortilla, sprinkle with chopped onions and cilantro and finish with a spritz of lime juice. Repeat with the remaining tortillas and pork.