Take the papery cover off the tomatillos and wash them to get the sticky stuff off. Slice in quarters and roughly slice the jalapeno*. Heat the oil in a big saucepan over medium heat and saute the tomatillos, jalapenos, onion and garlic for about 3 minutes. Make sure not to let the seeds burn. Add the salt, oregano and cumin and saute for another 2 minutes.
Meanwhile, cut the stems off the dried chiles and chop the guajillo into 1-inch pieces. Add them to the pot, along with the water and vinegar, after the spices have become fragrant. Cover and cook for about 10 minutes over low heat. Take the lid off and cook until everything is soft and most of the water is cooked off. You will need to keep enough liquid to blend it all together, about 1/2 cup.
Let the pot cool for about 15 minutes, until it is safe to throw in the blender. Puree it until smooth (you will see flecks of chile still), transfer to an air-tight container and store in the fridge. I imagine this would last several weeks, but it's never lasted that long in my house.
Makes about 2 cups.
*Be careful not to touch your eyes after you slice the jalapenos; you may want to wear gloves if you are sensitive to hot peppers.