There seems to be a lot of confusion over this subject. So let me just clear it up.
There is a leech-limit you CANNOT gain more than 5% of your own hp and mp per hit. So let's say for example, your HP is 3,000. This means the total maximum amount of HP you can hope to steal from a mob in 1 hit is 3,000 * 0.05 = 150. And your MP is 2,000. This means the total maximum amount of HP you can hope to steal from a mob in 1 hit is 2,000 * 0.05 = 100.
Now comes your max leech. This is given by your character level divided by 10, so let's say you are lvl 150. Your max leech will be 150/10 = 15.
To increase this limit, you must equip vampiric items. So let's say you have 2x vampiric earrings +4 equipped (+30 max hp steal per hit each), and a vampiric amulet +5 (+45 hp steal per hit), this is +105 max hp steal per hit total.
Now let's say we are wearing 1 extortion bracelet +4, this gives 4% hp steal of the total damage. So imagine we hit a mob and damage it for 5,000hp. We will gain 4% of this which = 5,000 x 0.04 = 200HP.
But as we are only wearing +105 max HP steal per hit from equipment and our base leech is 15 HP (lvl 150 divided by 10) we only gain 120hp from the mob.
I hope this guide helps.