Place dried chilies in a medium saucepan over medium high heat and cook, turning occasionally, until pliable and fragrant, about 1 minute. Add chicken stock and bring to a boil over high heat, reduce to a bare simmer, and let cook until chilies are totally softened, about 15 minutes. Transfer to the jar of a blender and wipe out pan.

Add 2 tablespoons of oil to pan and heat over medium-high heat until shimmering. Add onions and cook, stirring, until softened, about 5 minutes. Add garlic, cumin, coriander, and oregano and cook until fragrant, about 30 seconds. Add orange juice, raisins, chipotle chilies, and fish sauce and whisk to combine. Bring to a simmer, and then transfer mixture to blender with chilies and stock. Blend on high speed (start slowly and build up speed to avoid an explosion) until completely smooth, about 45 seconds. Season to taste with salt and pepper.

Carefully pat bison dry with paper towels or a clean kitchen towel. Heat remaining 2 tablespoons vegetable oil in a large heavy-bottomed Dutch oven over high heat until smoking. Add bison all at once and spread evenly over bottom surface. (It's ok if not all the bison is touching the bottom or if the pan is crowded.) Cook without moving until bottom surface is well browned, about 8 minutes.

Add chili mixture to Dutch oven and stir to scrape up any browned bits from the bottom. Add bay leaves. Bring to a boil, and then reduce to a bare simmer. Cover, leaving lid slightly ajar, and cook, stirring occasionally until bison is completely tender, about 3 hours. Season to taste with more salt and pepper, and serve with chopped cilantro, scallions, cheese, and sour cream as desired.