For the dressing: Whisk together all ingredients in a medium bowl. Set aside.
For the Salad: In a large pot of salted boiling water, cook pasta until very tender, about 15 minutes. Drain pasta and transfer to a large bowl. Add vinegar and stir until completely absorbed. Let pasta stand until cool, about 10 minutes.
Add dressing and stir until well coated. Stir in celery, shallots, and scallions. Season with salt and pepper. Cover and chill in refrigerator for at least 1 hour before serving.
