This is what I have come to think of how these health related attacks work... Test em and correct em maybe. I'll try and be a specific as possible. Any suggestions are welcomed. I have updated this everytime something has been tested and proven. This is accurate.

P1
Life leech is a life leech dmg type over time. It is resisted by life leech resistance. It is increased by +%life leech. This is true for both heros(our toons) and enemies. The amount of dmg that is inflicted on your enemy gets returned back to you as health.

P2
Vitality dmg is a vitality dmg type. It is not increased by any character stat(possibly increased by intellegence). It is resisted by vitality resistance. It is increased by +%vitality dmg. This is true for both heros(our toons) and enemies. 

P3
##% reduction of health is a vitality dmg type. It takes the total current health (like the static charge skill from the sorc in D2) value, determines the %reduction and inflicts that number. The number is then resisted by vitality resistances and for enemies there is also an additional resistance to specifically %reduction of health attacks. This is why this dmg has little to no effect on bosses. When using items that reduce resistances all resistances (including the hidden %health are lowered) This dmg can NOT be increase by +%vitality dmg. 


P4
% of Attack converted to health. This stat will take the "flat" dmg that you inflict (physical, pierce, fire, cold, lighting, vitality) (no dots). Once that dmg has been resisted by the armor and resistance of the enemy is it totaled, multiplied by the %conversion to health and then before this health is granted to you it is reduced by the life leech resistance of the enemy. The amount of health recovered can NOT be directly increased by +%life leech, but indirectly the amount CAN be increased by lowering the -life leech resistance of the enemy.