1/3 cup bottled balsamic vinaigrette
2 Tbsp spicy brown mustard
4 skinned, boned chicken-breast halves, trimmed of visible fat, each cut in half lengthwise, then crosswise in 1 1/4-in.-wide pieces
1 each yellow squash and zucchini (about 6 oz each), halved lengthwise, then cut diagonally in 1/2-in.-wide pieces
8 10 to 12 in. skewers
Whisk vinaigrette and mustard in a medium bowl until blended. Remove 2 Tbsp and set aside. Add chicken and both squashes to bowl; toss to coat.
Heat barbecue grill. While grill heats, alternately thread chicken and squashes on skewers, leaving small spaces between food. Discard dressing in bowl.
Grill 4 to 6 in. from heat source 12 to 13 minutes, turning once, until chicken is cooked through and squashes are crisp- tender. Drizzle or brush with reserved 2 Tbsp dressing.