Bring a large Dutch oven of salted water to a boil over medium-high heat. Add pasta, and cook 5 minutes. Add green beans, and cook 5 to 6 minutes or until pasta is tender but still firm to the bite and green beans are tender. Drain and rinse with cold water; place in a large bowl.
Add tomatoes, vinaigrette, and onion, tossing to coat. Serve immediately, or cover and chill up to 2 hours.