Marinate steak: Whisk together shallot, vinegar, mustard, sugar, 1 tsp salt, and 3/4 tsp pepper. Add 1/2 cup oil in a slow stream, whisking until emulsified.
Toss steak with 1/2 tsp salt, then marinate in a sealed bag with 6 Tbsp vinaigrette, chilled, turning bag occasionally, at least 2 hours. Chill remaining vinaigrette.
Make kebabs: Prepare grill for direct-heat cooking over medium-hot charcoal (medium-high heat for gas).
Meanwhile, toss tomatoes and okra with remaining 3 Tbsp oil and 1/4 tsp salt. Thread tomatoes onto 3 or 4 skewers. Thread okra crosswise onto pairs of parallel skewers, leaving small spaces between pieces. Put on a tray.
Thread steak onto remaining skewers, leaving small spaces between pieces (discard marinade). Put on another tray.
Oil grill rack, then grill steak skewers, covered only if using a gas grill, turning once, about 5 minutes total for medium-rare (about 8 minutes if using steak tips). Transfer to a platter. Drizzle with some of remaining vinaigrette while hot and let stand 5 minutes.
Grill tomato and okra skewers, covered only if using a gas grill, turning occasionally, until tomatoes just begin to wilt and okra is tender, 8 to 10 minutes total. Transfer to platter with steak and serve with any remaining reserved vinaigrette.
Cooks' notes: ·Steak can be marinated up to 24 hours. ·Steak and vegetable skewers can be grilled in a hot oiled large (2-burner) ridged grill pan.