Preheat oven to 325°F. Trim fat from brisket. In a small bowl, combine chili powder, cumin, and oregano. Sprinkle chili powder mixture evenly over all sides of the brisket; rub in with your fingers. Place brisket in a shallow roasting pan. Top with sliced onions; pour broth and vinegar over meat. Roast in the preheated oven, covered, for about 3 hours or until meat is fork-tender. Let meat stand in the cooking juices for 15 minutes. Remove meat. Using a slotted spoon, remove onions and set aside, reserving broth. Halve meat crosswise. When meat is cool enough to handle, use two forks to pull meat apart into shreds. Place shredded meat in a serving bowl. Stir onions into meat. Stir in enough of the reserved cooking liquid (1/2 to 1 cup) to moisten meat. If desired, use remaining cooking liquid to make Chile Enchilada Sauce.