You have two choices here. Either you can chop each breast into pieces and arrange them onto a shish with the vegetables, thereby assuring that each patron gets exactly one boneless breast (usually ½ lb of meat, precooked weight, of course) and a given volume of vegetables, or you can grill the breasts intact, starting them first and then adding the vegetables to the grill later, divvying out the portions at final serving. One takes more prep time, the other more time at the serving.
If you're serving a horde of famished carnivores directly from your grill, arrange the portions on skewers before you begin cooking; otherwise they'll wave away your attempts to put any vegetable matter on their plates.
