Heat a dry skillet over medium heat. Add the dried chiles one at a time and cook them for 20 to 30 seconds on each side, pressing down with a spatula, until soft, pliable, and slightly redder in spots. Don’t let the chiles burn; you’ll end up with a bitter sauce. Rinse the chiles, remove the stems, veins, and seeds, and tear each one into two or three pieces. Put them in a small saucepan with enough water to cover and cook at a very gentle simmer until well softened, about 20 min.
Meanwhile, in a saucepan or large skillet, heat 1 Tbs. of the vegetable oil, add the chopped onion, and sauté over medium-high heat until the edges of the onion are deeply browned, about 10 min. Add the garlic; cook for another 1 minute, and set the pan aside.
At the same time, toast the sesame seeds in a dry skillet set over moderately low heat until they’re a deep golden brown. Stir the seeds frequently to keep them from burning and pour them onto a plate to cool as soon as they’re fully toasted.
Drain the chiles (discard the cooking water) and put them in a blender. Add the sautéed onion and garlic, sesame seeds, tomatoes, oregano, cumin, and 1 cup of the broth or water. Blend until completely smooth, about 2 minutes. You should have a medium thick paste; if it’s too thick, thin it with a little broth or water.
Wipe clean and reheat the skillet you used to sauté the onions. Add the remaining 1 Tbs. vegetable oil. When the oil is hot, add the paste from the blender. Cook the paste for 3 min., stirring constantly with a whisk or wooden spoon. Stir in the remaining broth, vinegar, bay leaves, and salt. Bring to a simmer and cook until the sauce begins to thicken, about 20 min. Discard the bay leaves. Taste and adjust the seasoning with more salt if needed. The sauce should still be a bit thin; it will thicken further when the enchiladas are cooked. Let cool slightly before making the enchiladas.