In a small bowl, whisk together 3 tablespoons sesame oil, the vinegar, soy sauce, garlic, ginger and crushed red pepper.
In a pot of boiling, salted water, cook the pasta until al dente; drain.
Heat a large skillet over medium heat. Add the walnuts and toast, 2 to 3 minutes. Transfer to a bowl, wipe out the skillet and heat the remaining 1 tablespoon sesame oil over high heat. Add the onion and asparagus and cook for 2 to 3 minutes. Add the sugar snap peas and the sesame oil sauce; cook for 1 minute over high heat. Lower the heat to medium, add the pasta and cook, stirring, until heated through, 2 minutes. Top with the walnuts.