Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x13-inch baking dish.
Melt butter in a large skillet over medium-heat. Cook and stir chopped bacon in hot butter until almost crisp, 5 to 10 minutes.
Pulse onion, carrot, celery, red bell pepper, poblano pepper, mushrooms, and garlic in a food processor until finely chopped. Add vegetable mixture and rosemary to bacon in the skillet; cook and stir until vegetables soften and sweeten, about 5 minutes.
Mix vegetable mixture, bread crumbs, and milk together in a bowl. Let cool to room temperature. Stir egg, salt, Worcestershire sauce, black pepper, and cayenne pepper into vegetable mixture. Add buffalo meat to vegetable mixture and mix with your hands until well-blended.
Turn meat mixture out into prepared baking dish and shape into a 9x5x3-inch meatloaf. Lay bacon strips crosswise over the top of the meatloaf, tucking in ends of bacon under the loaf.
Whisk rice vinegar, brown sugar, and mustard together in a bowl until glaze is smooth.
Bake meatloaf in the preheated oven for 30 minutes. Brush glaze over loaf and continue baking until no longer pink in the center, about 30 minutes more. An instant-read thermometer inserted into the center should read at least 155 degrees F (68 degrees C). Cool for 10 minutes before slicing.
