Light a grill. Brush all of the vegetables with vegetable oil and lightly season with salt and pepper. Grill the tomatoes, onion, zucchini and yellow squash over moderate heat, turning once, until the vegetables are charred and just tender, about 3 minutes per side. Grill the red bell pepper on 4 sides until charred all over, about 3 minutes per side. Peel the tomatoes and red bell pepper. Cut all of the vegetables into 1/2-inch dice.