Grind the pork and the fat in a meat grinder.
Blend 1/2 cup of the vinegar with the garlic, oregano, cumin, and coriander in a blender until the garlic and spices are finely ground. Add the rest of the vinegar, the poblano chiles, and the ground pasilla chile and blend until the chiles are finely ground. Gradually add the cilantro, parsley, and Swiss chard, and blend until smooth.
Mix the ground pork with the puree and the salt; mix well to evenly distribute the ingredients. Place in a bowl, cover, and refrigerate for at least 2 hours, ideally overnight.
Heat an 8-inch sauté pan. Fry a small patty of the sausage for 2 to 3 minutes on each side. Taste for seasoning, and add more salt if necessary.
Rinse casing under cold water. Soak casing in 3 cups water mixed with 1 Tbsp vinegar. Cut the sausage casings into approximately 2-foot lengths. Tie a knot in one of the ends. Place some of the sausage mixture in a pastry bag (not too much or it will be impossible to pipe) and pipe into sausage casings. Smooth out the filling in the casing to evenly distribute the meat; do not overstuff the casings or they will break. Twist into 4-inch lengths.
To cook: Prick the chorizo several times with a fork. Heat a large cast-iron skillet or a 10-inch sauté pan. Over medium heat, evenly brown the chorizo, approximately 8 to 10 minutes. Serve with warm tortillas to make tacos.