Title almost self explanatory. I wanna know if flat damage you receive from sources other than shields also applies to skills with % off-hand weapon damage? Also, flat damage on shields (except the physical part, which I presume not) apply on main hand weapon damage just like any other flat damage? Meat shield, for example, has the normal shield physical damage plus flat vitality damage. Would this flat vitality be applied through % main-hand weapon damage skills? Or only off-hand weapon damage? And the other way around, as I mentioned. 
Any flat damage mods, or Weapon Effects, present on weapon, will apply only to attacks made with this weapon. If they’re present not on Weapon (while you have weapons in both hands), they will work for both weapons. Shield is considered a weapon too, it just isnt used for normal attack and most skills. Some skills, however, use shield as weapon, and then, its mechanics is the same to normal weapons.
So, any Weapon Effect or flat damage on shield work only for attacks with shield. Weapon effects and flat damage from other sources do apply to attacks with shield as well.