In a 2 quart saucepan over medium heat, add the Ancho and Guajillo dried chiles, vinegar, broth, sugar, salt, and a very small amount of water if desired. Let cook until the chile skin is soft.
Then, place the mixture in a blender, add ¼ cup chopped onion and garlic and blend on High for 1 to 2 minutes or until mixture has a thick and smooth consistency. Place the pork in slow cooker and pour the chile sauce over the pork. Add the bay leaves to the slow cooker.
Cook on LOW for about 6 to 7 hours or until the pork is tender and thoroughly cooked. Remove the bay leaves. Transfer the pork to a medium-size bowl. Shred the meat by using two forks and pulling them away from each other.
In a small bowl, mix the sour cream and milk, stirring constantly for 1 minute. Set aside. Place the tortillas on a serving dish. Spoon the pork, cilantro, and finely chopped onion over the tortillas. Drizzle with the sour cream sauce. Serve with lime wedges on the side.