To prepare sauce:
On a heavy ungreased griddle or skillet set over medium heat, toast chiles, a few at a time, pressing them down with a spatula for a few seconds, then flipping and pressing again; be careful not to burn the chiles or the sauce will be bitter. Transfer the toasted chiles to a bowl, cover with boiling water, weight with a plate and let soak for 30 minutes. Drain. Meanwhile, roast garlic cloves on the same griddle or skillet, turning them often, until soft and blackened in spots, about 15 minutes.
Cool, peel and chop. Pulverize the cumin and oregano in a spice grinder or mortar and pestle , then place in a blender, along with the drained chiles, garlic, and 1 cup of the beef stock. Blend until smooth, then strain through a medium-mesh sieve. Heat a wide, large non-stick saucepan over medium-high heat. When hot enough to make a drop of the puree sizzle, add puree all at once. Reduce heat to low, and simmer, stirring regularly, until the mixture reduces to a dark, thick puree, 10-15 minutes. Add 1-1/2 cups stock, partially cover and simmer, stirring frequently to prevent scorching, about 45 minutes. Season with sugar and salt. Thin with a little extra stock, if necessary, to give the sauce the consistency of heavy cream. The sauce can be made ahead and stored, covered, in the refrigerator for up to 2 days.
To marinate and cook beef:
Combine vinegar with 3 tablespoon of the sauce. Brush the mixture over beef. Place the meat in a shallow dish, cover with plastic wrap and refrigerate for at least 2 hours, preferably overnight. Prepare a charcoal fire or preheat a gas grill. Brush the meat with oil and grill with the cover on, turning the meat regularly, to desired doneness--120 degrees F for rare, 145 degrees for medium. Remove to a rack set over a plate and let rest in a very low oven for 10 minutes before serving. Just before serving, reheat the sauce and slice the meat. Spoon the sauce onto warm dinner plates and top with the beef.