COOK pasta as directed on package, omitting salt; drain, reserving 1/4 cup cooking water.
MEANWHILE, heat dressing in large skillet on medium-high heat. Add zucchini, peppers and onions; cook and stir 8 to 10 min. or until crisp-tender. Add tomatoes; cook and stir 2 min. Remove from heat.
ADD pasta, reserved cooking water and mayo to skillet; mix well.