Generously sprinkle the chicken with some salt and pepper. Let sit for 10 minutes, and then transfer to a large resealable plastic bag. Combine 1 cup of the apple cider vinegar, the vegetable oil, Worcestershire sauce, and garlic in a large glass measuring cup. Pour the mixture over the chicken and seal the bag, squeezing out as much air as possible. Place the bag in the refrigerator and allow the chicken to marinate for at least 2 hours and up to 8 hours.
Meanwhile, prepare the sauce. Combine the remaining 1 cup apple cider vinegar, the marmalade, mustard, sugar, salt and crushed red pepper flakes in a medium saucepan over medium-high heat. Bring to a simmer and cook until the sugar dissolves and the mixture is slightly thickened, about 10 minutes. Set aside until ready to use.
Prepare an outdoor charcoal grill to medium-high heat, placing the coals on one side of the grill for direct and indirect heat.
Remove the chicken from the marinade and pat dry. Grill the chicken on the direct heat until charred on all sides, about 8 minutes per side, turning and basting often with the sweet & sour BBQ sauce until it becomes caramelized and the internal temperature of breast meat registers 165 degrees and legs, thighs, and drumsticks register 175 degrees, about 5 minutes. Transfer chicken to platter and let rest 10 minutes. Serve with remaining sauce on the side.