Cook the pasta in plenty of rapidly simmering water until al dente. Once it's just about done, plunge the broccoli, carrots, and squash into the water and cook for another minute or so.
Drain and rinse the pasta and vegetables under cool running water in the colander, then drain well again.
Combine the pasta and vegetable mixture and all the remaining ingredients in a large serving bowl, and toss well. Let stand for a few minutes, then taste to see if you need more vinaigrette, lemon juice, and/or salt and pepper.
Serve at once or cover and refrigerate until needed.