Combine the vinegar, oil, pepper, garlic powder and oregano to make a marinade.
Cut the beef into 1-1/2" cubes. Cut the onions into quarters; cut the bell peppers into 1-1/2" squares.
Place the meat and vegetables in a covered container and marinate for at least 30 minutes.
Load the skewers, alternating with meat and vegetables.
Grill kebabs over medium heat for 10 to 30 minutes, depending on desired degree of doneness.