According to the GW2 wiki, Falling Damage:
Fall damage is proportional to the characters health: if a level 80 and a level 1 fall the same height, the amount taken will be different, but the percentage of health taken away will be the same.
Does this mean that both Toughness and Vitality don't reduce fall damage? Or would Toughness help because it's a fixed damage reduction rather than an increase in HP?