Hi, depending on the imbue you have you will receive 5%, 10% or 25% of the damage you deal to all the monsters you hit with the runes. So this means its not based on the on the total HP the monster has but entirely on the damage you specifically hit with each attack. Being 5% for Basic Vampirism, 10% for Intricate Vampirism and 25% for Powerful Vampirism.
Edit:
After going through a field research I think I now have the answer to your question, I used an armor with powerful vampirism and tried on different amount of monsters of the same kind (Dragon Lords), wearing the exact same gear and hitting them with avalanche runes (No wand on) these were the results:
4 Dragon Lords:
05:21 A dragon lord loses 438 hitpoints due to your attack.
05:21 You were healed for 36 hitpoints.
05:21 A dragon lord loses 438 hitpoints due to your attack.
05:21 You were healed for 36 hitpoints.
05:21 A dragon lord loses 438 hitpoints due to your attack.
05:21 You were healed for 36 hitpoints.
05:21 A dragon lord loses 438 hitpoints due to your attack.
05:21 You were healed for 36 hitpoints.
3 Dragon Lords:
05:33 A dragon lord loses 383 hitpoints due to your attack.
05:33 You were healed for 39 hitpoints.
05:33 A dragon lord loses 383 hitpoints due to your attack.
05:33 You were healed for 39 hitpoints.
05:33 A dragon lord loses 383 hitpoints due to your attack.
05:33 You were healed for 39 hitpoints.
2 Dragon Lords:
05:32 A dragon lord loses 387 hitpoints due to your attack.
05:32 You were healed for 54 hitpoints.
05:32 A dragon lord loses 387 hitpoints due to your attack.
05:32 You were healed for 54 hitpoints.
1 Dragon Lord:
05:29 A dragon lord loses 438 hitpoints due to your attack.
05:29 You were healed for 110 hitpoints.
As you can see the amount of hp that I regain increases. This means the more monsters I hit the less hp i leech back from creatures. Analysing the numbers this was the result:
1 Dragon Lord= 25% life leech
2 Dragon Lords= 14% life leech
3 Dragon Lords= 10.1% life leech
4 Dragon Lords= 8.2% life leech
About the relation between percentage/amount of monsters I think it has to do with the type of monster your attacking, its weaknesses, the runes you use, ml of the character, etc etc. But I'd have to try on different monsters, with different runes to come up with a reason for that difference, though I think I already answered your question :)