Look for aji amarillo paste, a spicy Peruvian seasoning made from yellow chile peppers, in Latin markets. It's wonderful in marinades and meat rubs.
Place 1/4 cup vinegar, aji amarillo paste, 2 teaspoons cumin, black pepper, salt, and garlic in a mini food processor; process until smooth. Place marinade in a large zip-top plastic bag.
Cut chicken into 2-inch cubes. Place chicken cubes in bag with marinade; toss well to coat. Marinate in refrigerator at least 45 minutes or up to overnight.
Peel onion; cut into 1-inch wedges. Remove stem, seeds, and membrane from bell pepper; cut into 1-inch pieces.
Remove chicken from marinade; discard marinade. Thread the chicken and vegetable pieces evenly onto 8 (10-inch) skewers, alternating chicken with vegetables.
Combine remaining vinegar, remaining cumin, oil, dash of salt, and dash of pepper in a small bowl.
Preheat grill to medium-high. Place skewers on grill rack coated with cooking spray; grill skewers 2 minutes on each side or until chicken is done, basting frequently with vinegar mixture.
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