In a saucepan, combine the olive oil, vinegar, garlic, red pepper flakes, black pepper, and cayenne pepper. Cook on low heat for about 5 minutes, stirring occasionally.

Add the mango to the mixture and continue to cook on low heat for about an additional 5 minutes. Stir occasionally and remove from heat when mixture thickens.

Place the steak in a glass dish and pour the mango marinade over the steak. Cover the dish and place in the refrigerator for at least one hour.

Thread skewers with the marinated steak, bell peppers, onion, and tomatoes. Place skewers on a hot grill and cook until done.