Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs, adding the snap peas and bok choy during the last 2 minutes. Reserve 1/2 cup of the cooking water, then drain. Transfer the pasta and vegetables to a large bowl.
Meanwhile, pulse the cashew butter, 2 tablespoons each water and rice vinegar, the Sriracha, scallions, ginger, 1/4 cup cilantro and 1/2 teaspoon salt in a food processor until smooth. Toss the bell pepper and radishes in a medium bowl with the remaining 1/4 cup cilantro and 1 tablespoon rice vinegar.
Add the sauce to the pasta mixture along with enough of the reserved pasta cooking water to loosen; toss to coat. Top each serving with the diced vegetables and some Sriracha.