Cook pasta in boiling salted water until tender. Drain in a colander then immerse in cold water until pasta is cool. Drain thoroughly and place in a bowl with the vegetable oil. Toss until pasta is coated evenly with the oil. Cover and refrigerate while preparing the vegetables.
Place tomatoes, red onion, and celery in a bowl with a tight-fitting lid. Whisk together vinegar, sugar, garlic, dry mustard, salt, and pepper. Pour over vegetables, cover bowl, and let marinate for several hours in the refrigerator.
Drain the marinated vegetables, reserving the liquid. Combine vegetables, cooked pasta, peas, raisins, and sunflower seeds in a large bowl.
Whisk the mayonnaise into the reserved marinade and pour over salad. Toss gently to mix. Cover and chill until ready to serve.