1. In a food processor, puree cashews, scraping sides and bottom of bowl with a spatula, until smooth. Add yogurt, lemon juice, and garlic; pulse until blend well. Transfer to a bowl; salt and pepper to taste.
2. Preheat broiler and lightly oil 2 large shallow baking pans. Arrange eggplant slices, in a single layer, in prepared pans. Lightly brush tops with some oil; season with salt and pepper. Broil, turning once, until tender and golden brown, about 6 minutes; transfer to a plate. Repeat with bell peppers, zucchini, onion, and remaining oil in same manner.
3. Arrange pitas on a baking sheet and broil (being careful not to burn), turning once, until pale golden, about 2 minutes. Transfer to rack to cool slightly, about 5 minutes.
4. Spoon some cashew spread into pitas, spreading it on both sides, then fill each with broiled vegetables, tomato, and basil. Serve sandwiches, standing up, in a basket or bowl. Makes 6 servings.