Bring a large pot of water to a boil and fill a bowl with ice water. Add the bean sprouts to the boiling water and blanch for 10 seconds. Using a slotted spoon, transfer the bean sprouts to the ice water to cool, then drain and pat dry. Fill the bowl with more ice water. Add the rice vermicelli to the pot and cook until tender, about 5 minutes. Drain the noodles and transfer them to the ice water to cool. Drain again, shaking off the excess water; pat dry with paper towels. Wipe out the bowl. Cut the noodles into 4-inch lengths and return them to the bowl. Add the bean sprouts, cucumber, mint and scallions.