Cook pasta in a large pot of boiling salted water until al dente, according to package instructions. Reserve 1/2 cup pasta water. Add snow peas, carrots, and tofu to pot; immediately drain pasta mixture, and set aside.
In pasta pot, stir together peanut butter, soy sauce, vinegar, and sugar. Add reserved pasta mixture; toss gently, adding reserved pasta water a little at a time to create a thin sauce that coats spaghetti (you may not need all the water). Season as desired with salt and pepper. Serve.