Preheat a grill to medium high. Bring a large pot of salted water to a boil. Whisk the rice vinegar, soy sauce, honey and sesame oil in a large bowl. Whisk in the vegetable oil. Spoon 2 tablespoons of the dressing into a small bowl.
Add the soba noodles to the boiling water and cook as the label directs. Drain and rinse under cold water; set aside. Season the steak with salt and pepper and brush all over with the reserved 2 tablespoons dressing. Season the red onions and squash with 1/4 teaspoon salt and a few grinds of pepper.
Grill the steak until medium rare, 3 to 5 minutes per side; let rest 10 minutes. Grill the red onions and squash, turning as needed, until charred and tender, about 6 minutes. Thinly slice the steak against the grain and season lightly with salt. Chop the onions and slice the squash.
Add the noodles, onions, squash, radishes and herbs to the bowl with the dressing; drizzle with any accumulated juices from the steak and toss. Divide among bowls. Top with the steak.