Melt butter in a nonstick skillet over medium heat. Add onion, garlic, and ginger; sauté 6 minutes or until tender. Add curry and red pepper. Sauté 2 minutes. Stir in vinegar. Cool 10 minutes. Combine onion marinade and chicken in a heavy-duty zip-top plastic bag; seal and chill for 2 hours.
Combine chicken, marinade, and 1 cup water in a large Dutch oven over medium-low heat. Bring to a boil. Cover, reduce heat to low, and simmer 20 minutes, stirring occasionally. Add potato; bring to a boil. Cover, reduce heat to low, and simmer 20 minutes or until potato is tender. Serve over rice. Garnish, if desired.