1. In a small bowl, mix coriander, cumin, cayenne, turmeric, and vinegar until smooth.
2. Rinse chicken and pat dry. Rub spice mixture all over breast halves.
3. Pour oil into a 10- to 12-inch nonstick frying pan over high heat, swirling to coat bottom. When oil is hot, add chicken and turn as needed to brown on both sides, 4 to 5 minutes total. Transfer to a rimmed plate.
4. Add onion, garlic, and ginger to pan; stir often until onion is lightly browned, 2 to 3 minutes. Add coconut milk and 1/2 teaspoon salt; stir often until mixture boils. Add chicken and any accumulated juices, reduce heat, and simmer uncovered, turning pieces once, until no longer pink in center of thickest part (cut to test), 10 to 12 minutes total. Transfer chicken to a rimmed platter.
5. Turn heat to high and boil sauce until reduced to 1 1/2 cups, about 4 minutes. Spoon evenly over chicken. Add more salt to taste, and garnish with pepper rings if desired.