Place green beans in a large bowl. Boil potatoes and 1 teaspoon salt 5 minutes, or until tender. Drain through a sieve over the beans, and let stand 5 minutes. Drain beans; add potatoes, bell peppers, 1/2 teaspoon salt, and black pepper to taste. Toss and divide evenly among 4 plates. Top each portion with 1/3 cup tuna and 1 tablespoon drained olives. Sprinkle with 1 teaspoon vinegar.