In a large skillet cook leeks in oil over moderately low heat, stirring, until softened, about 10 minutes. Stir mushrooms and cook 5 minutes. Stir in scallions, soy sauce, and vinegar and cook 1 minute. In a kettle of salted boiling water cook noodles 5 minutes or according to package directions. Put pea shoots in a colander and drain cooked noodles over shoots to wilt them. Rinse mixture in cold water and drain well. In a bowl toss noodles with pea shoots and stir in cooked vegetables. Season mixture with salt and pepper and serve at room temperature.