In a large pot of boiling, salted water, cook the pasta until al dente; drain and spread onto a baking sheet to cool.
Meanwhile, pour the vinegar into a large bowl and whisk in the olive oil in a slow, steady stream. Stir in the tomatoes, cucumber, red onion and olives. Add the pasta and parsley and toss; season with salt and pepper. Stir in the tuna. Top with the feta.
