Arrange eggplant in 1 layer on a large baking sheet and brush both sides with 2 tablespoons oil (total). Broil about 4 inches from heat, turning once, until golden, 8 to 12 minutes total.
Stir together vinegar, mint, capers, ¼ teaspoon salt, ½ teaspoon pepper, and remaining 3 tablespoons oil and toss with warm eggplant. Marinate at least 20 minutes.
Notes
Marinated eggplant can be made 1 day ahead and chilled. Bring to room temperature before serving.