Cook pasta according to package directions, omitting salt and fat. Add squash and zucchini during the last minute of cooking. Drain pasta mixture; rinse under cold water.
Heat a large nonstick skillet over medium-high heat until hot. Add prosciutto; cook 5 minutes or until crisp, stirring frequently.
Combine the pasta mixture, prosciutto, onion, and cheese in a large bowl; sprinkle with salt and pepper. Combine white balsamic vinegar, olive oil, Dijon mustard, and rosemary in a small bowl, stirring with a whisk. Add vinegar mixture to pasta mixture, tossing gently to coat.