Combine marinade ingredients and place in a shallow bowl.
Add the flank steak, cover, and marinate in the refrigerator for 4 hours, or over night, turning once.
Light the grill or preheat the broiler.
To make the sauce: Place the water, garlic, fish sauce, lime juice, sugar substitute, and red pepper flakes in a food processor or blender. Pulse until well combined; set aside.
Place the noodles in a deep bowl. Cover with warm water and let soak for 10 minutes. Drain and then boil in fresh water to cover for 2 to 4 minutes, until cooked through. Drain, rinse under cold running water, and drain again. Set aside.
Remove the steak from the marinade. Score the meat in a cross-hatched pattern and grill until done, about 8 minutes per side for medium.
Transfer steak to a carving board and let rest 10 minutes before slicing into very thin slices on a diagonal.
To assemble the salads, divide the greens, cucumber, and herbs between 4 dinner plates.
Mound the noodles in the center of each plate and top with an equal portion of steak slices.