Using this mod is recommended if one wishes to withstand significant damage from Slash and Toxin effects.
This synergizes best with Steel Fiber along with a Warframe of more than 125 Base Health or with a Warframe with a high armor rating, such as Valkyr.
Vitality is the one of the most common mods to ever drop in the game. Stockpiling such will be a decent alternative in levelling mods instead of using Fusion Cores or as a source of income. The duplicates can also be used to level up Vitality in case of insufficient quantities of Fusion Cores available, though doing so can be credit-expensive.
The increase is applied to the base (rank 0) health of the Warframe, before health from leveling is taken into account.
Example: the base Health of a rank 0 Frost Warframe is 100. Once at rank 30, the warframe will have total health of 300, but will only gain 40 health from a standard Vitality mod since its base health is 100. The final result for a rank 30 Frost with a rank 0 Vitality mod will be (100)+(100*40%=40)+(100*200%=200)=340 health.