1. Preheat oven to 375°.
2. Combine vinegar, sugar, and salt in a bowl, stirring until sugar dissolves. Add carrot; let stand 15 minutes, stirring occasionally. Drain.
3. Heat a grill pan over medium-high heat. Cut eggplant stem off. Cut eggplant lengthwise into 1/4-inch-thick slices; brush with oil. Grill 7 minutes or until tender, turning once.
4. Combine peanut butter, onions, ginger, miso, and juice in a bowl; stir well.
5. Hollow out top and bottom halves of bread, leaving a 1-inch-thick shell; reserve torn bread for another use. Place bread on a baking sheet, cut sides up. Bake at 375° for 5 minutes or until golden brown. Spread bottom half of bread with peanut butter mixture. Arrange eggplant evenly over peanut butter mixture. Arrange carrot mixture and cucumber evenly over eggplant; top with cilantro and jalapeño, if desired. Place top half of bread on sandwich. Cut into 5 equal pieces.