In a small bowl, microwave the potatoes on high for 4 minutes.
In a large bowl, whisk together 1/2 cup plus 2 tablespoons olive oil, the vinegar, garlic, 1/2 teaspoon salt and 1/4 teaspoon pepper. Add the eggplant, bell peppers, onion and partially cooked potatoes and toss to coat. Marinate at room temperature for 30 minutes.
Meanwhile, in a medium pot of boiling salted water, cook the orzo until al dente, about 8 minutes. Drain; return to the pot. Add the tomatoes and the remaining 2 tablespoons olive oil. Cover to keep warm.
Preheat a grill or grill pan to medium. Thread the marinated vegetables onto 8 skewers; reserve the marinade. Grill the kebabs, turning occasionally and basting with the reserved marinade, until tender and slightly charred, 12 to 15 minutes. Serve with the orzo.