Bring a pot of water to a boil; season generously with salt.
In a small bowl, combine soy sauce, sesame oil, vinegar, ginger and garlic; season with salt and pepper; mix well.
Once the water boils, cook the pasta according to package directions until al dente; reserve ¾ cup pasta cooking water; then drain.
Transfer pasta to a large bowl; add half the dressing and ¼ cup pasta water; season with salt and pepper; toss to evenly coat.
Meanwhile, in a large deep-sided saucepan, over medium heat, heat the safflower oil; add onion and bell peppers; season with salt and pepper; cook, stirring occasionally, until deep golden and tender; about 10 minutes.
Add snap peas; cook, stirring occasionally, until crisp-tender; 2 minutes.
Add tomatoes, ¼ cup pasta water and the remaining dressing.
Stir to deglaze the pan and lift the browned bits on the bottom of the skillet; then add vegetable mixture (with sauce) and scallions to pasta; toss until well combined (add more pasta water if thinner sauce is desired); season with salt and pepper.
Serve warm or at room temperature.
Garnish with scallions and sesame seeds, if using.