Heat a pot that can hold the chicken in one layer over medium heat.
Add the vegetable oil and cook, skin side down for about 4-5 minutes.
Turn the chicken over and cook an additional 2 minutes. Remove chicken and set aside.
Spoon off all but about 3 Tbsp of fat.You need this much because your spices will burn otherwise.
Reduce heat to medium low and add the onion and garlic and cook until the onion is softened.
Add the chili powder, cumin and oregano and cook 2 minutes. Watch to make sure the spices do not burn.
Turn the heat back up to medium. Add the tomatoes and tomatillos and cook for about 3-4 minutes, stirring frequently. Watch that it doesn't dry out.
Add the cider vinegar and cook a minute more.
Return the chicken and accumulated juices. Add enough chicken stock to almost cover.
Reduce heat to low and simmer for 5 minutes.
Turn the chicken over and cook until an instant read thermometer reads 165F. This will take 5-10 minutes.
Remove chicken and set aside. Turn the heat up to medium high and reduce by about half.
Add the chipotle and adobo sauce and puree. If you are using a blender, take the thing in the middle of the lid off and cover with a towel. Unless you enjoy cleaning tomato sauce off the ceiling.
Taste for salt at this point and adjust as required.
When the chicken has cooled enough to handle, shred coarsely. Two forks works well here. Your hands work better.
Combine the shredded chicken with the pureed sauce and warm over low heat.