Vaal Pact I'm confused

As I understand it without Vaal Pact your leech rate is 20% of your max life.

So if you have 2000 life your max leech rate would be 400 per sec.

Vaal pact says "Life leech applies instantly at 40% effectiveness". So from that I am expecting, for 2000 life, 800 per sec.

But the wki says the amount leeched is reduced. I quote (my bold)

"Without Vaal Pact, life leech occurs at a base rate of 20% of maximum life per second. The duration is the leech amount divided by the leech rate. Multiple hits do not stack; only the fastest leech at any point in time will recover life. The maximum leech rate is thus 20% of maximum life per second. With Vaal Pact, the amount leeched is reduced to 40%, but the leech is instant, so the full amount will be leeched regardless of timing or magnitude of hits. The amount leeched is thus only limited by damage dealt."

So what in reality does Vaal pact do please.

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AnyNick wrote:

As I understand it without Vaal Pact your leech rate is 20% of your max life.

So if you have 2000 life your max leech rate would be 400 per sec.

Vaal pact says "Life leech applies instantly at 40% effectiveness". So from that I am expecting, for 2000 life, 800 per sec.

But the wki says the amount leeched is reduced. I quote (my bold)

"Without Vaal Pact, life leech occurs at a base rate of 20% of maximum life per second. The duration is the leech amount divided by the leech rate. Multiple hits do not stack; only the fastest leech at any point in time will recover life. The maximum leech rate is thus 20% of maximum life per second. With Vaal Pact, the amount leeched is reduced to 40%, but the leech is instant, so the full amount will be leeched regardless of timing or magnitude of hits. The amount leeched is thus only limited by damage dealt."

So what in reality does Vaal pact do please.

It does exactly what it says.

First, your leech effectiveness will be only 40%. That means, if you have, for example, 10% life leech, then it will be reduced to effective 4% by Vaal Pact (if you have 20% life leech, it will be reduced to 8%, and so on).
Second, your life leech will apply instantly. When you deal a hit for 1000 damage while having 4% life leech (this is your "base" 10% reduced to 4%), then you will immediately recover 40 life.

In fact, that means, that your life leech will STACK from multiple hits.
Let's say, you get 20% "base" life leech, deal 1000-5000 damage per hit (crit/noncrit). Then, you will recover 80-200 hp every time you hit an enemy. And you make around 5-25 hits per second (quite possible with AoE skills). So, you will recover in average 2100 hp per second. Let's assume, your life pool is 5000. Then, getting Vaal Pact will be equal to getting around +160% "increased life leech rate". But as your DPS or number of targets you hit, or your Life leech values increase, the effectiveness of Vaal Pact will increase further. However, it wont increase as your maximum life increases.

So, you should take Vaal Pact when you deal damage in many small hits, have high Life Leech values and high DPS (compared to maximum life), and dont have high life regeneration.

Note also, that damage reflection occurs AFTER instant leech from Vaal Pact, so if you're on full life, you'll take damage and recover nothing. Note also, that if you leech more life than take damage reflected, then you will never be killed by reflected damage (except for case where you get "oneshot").

Thank you for that detailed explanation it makes sense.

One more question if I may.

Currently running a blood magic (gear derived) witch. She has 5K health and does 8.3K arc dps and 4.7K flame totem dps. Have 8.8% life leech gems on both.

If I have the totem active whilst ARCing will I get the stats you quoted for each separately or one be ignored or will it be treated as 13K dps.

Would running Vaal Pact be recommended for this Witch?

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AnyNick wrote:

Thank you for that detailed explanation it makes sense.

One more question if I may.

Currently running a blood magic (gear derived) witch. She has 5K health and does 8.3K arc dps and 4.7K flame totem dps. Have 8.8% life leech gems on both.

If I have the totem active whilst ARCing will I get the stats you quoted for each separately or one be ignored or will it be treated as 13K dps.

Would running Vaal Pact be recommended for this Witch?

Totem leeches life for itself, not for you. If you support totem with Life Leech gem, it will heal itself while dealing damage to enemies. Same for summons, etc
And as far as i know, Vaal Pact affects YOU, but not your totems or minions. So, they'll use "normal" life leech rules.

And i dont recommend running Vaal Pact with just 9% leech and just 9k DPS. You may try it, of course, but i think, it will make your life worse, not better.
To benefit from Vaal Pact, you should have more life leech, more DPS and more hits per second. Arc hits repidly, but compared to other skills (like CoC ones), it's no match at all.
Also, having life regeneration is highly recommended for Blood Magic.

P.S.
Another way to understand Vaal Pact:
"Life leech is applied instantly" is equal to "grants +999999999999999999999999999999999999999999999% increased life leech rate" (technically infinite).
So, any bonuses to life leech rate are useless with Vaal Pact.

Understand. Thanks once again for your help.