In a large bowl, cover the sliced onion with cold water and soak to mellow the flavor, about 15 minutes; drain.
Meanwhile, prepare the vermicelli according to package instructions; drain and rinse under cold water. Add the vermicelli and cucumber to the bowl with the onion; toss.
Preheat the broiler. On a foil-lined baking sheet coated with cooking spray, arrange the salmon fillets, spacing them a few inches apart. In a medium bowl, whisk the hoisin sauce, soy sauce, vinegar and crushed red pepper for the glaze. Drizzle about 1 tbsp. over each fillet.
Broil the salmon, rotating the baking sheet halfway through cooking, until the glaze is bubbling and the fish is cooked through, 5 to 7 minutes.
Drizzle the remaining glaze on the noodle mixture. Using a fork, break each salmon fillet into large pieces and place on top of the noodles.