Whisk vinegar, oil, mustard, oregano, rosemary, garlic, salt and pepper together in a small bowl.
Skewer beef, mushrooms, tomatoes, bell pepper pieces and onion chunks, alternating evenly, on 8 metal or wooden skewers. Place the kebabs in a 9-by-13-inch baking dish and pour the marinade over them. Refrigerate (or store in a cooler packed with ice) for at least 2 hours and up to 8 hours.
Preheat grill to medium-high. Remove the kebabs from the dish; discard the marinade. Grill the kebabs, turning once, to desired doneness, 6 to 8 minutes total. To grill over your campfire, hold the skewers over the flames (but do not let the flames touch the food), turning regularly, until the meat is cooked to your liking, about 15 minutes for medium.