Vitality Points Consumed = ( ( (Exp) / (Level^2) )*100 ) / 9
Exp = Base exp gained from mob
Level = Mob level
Example 1 1xHP Mob
Bone Grinder 12,903xp Level 70
( ( (12,903) / (70^2) )*100 ) / 9
Vit Consumed = 29 points.
Example 2 5xHP Mob
Karik 32,922xp Level 70
( ( (32,922) / (70^2) )*100 ) / 9
Vit Consumed = 75 points.
This means even if you only gain 1 exp point your vitality 'points' will still change. If you kill a gremlin at level 80 and gain 1 exp point you will lose the same number of vitality points as you would if you were only level 1.
In the example, the Karik has approx. 5x the HP as the Bone Grinder and removes 2.5x the vitality and only gives 2.5x the experience. With this said, unless you're in a full group for the XP bonus, don't hunt anything with any multiplier - go for the 1x mobs if you're specifically looking to make good use of your vitality.