You mustn't mix up life steal and spell vamp.
Life steal steals on physical damage (meaning auto-attack for vlad). In fights, you very rarely use your auto-attacks to deal big damage because you're a caster.
Spell vamp steals on magical damage (meaning all of his spells), this is want you want if your plan is to steal more life.
- Zhonya's Hourglass is great because vlad's ult has low range but must be cast in battle engage to hit the most enemies and for the 15% extra magical damage. The two second invulnerability will allow you to stay alive in the heart of the battle if you get targeted.
- I also very often use Hextech Revolver with Vlad, but you need some AP. It's no use having 50% Spell vamp if you don't deal any damage.
Vlad spells cost life, the good thing about more spell vamp is you can get your life back faster with transfusions minion lines and you can cast life costing spells (specially Tides of Blood that stacks) with less impact on your life.
EDIT : I went to have a quick look at all the different Vlad builds. All (or nearly all) of the descent builds have hextech revolver in them with its evolution to will of the ancients - give more AP, more spell vamp and an aura :-)
First, let's clear up a misconception.
Armor Penetration gives you a constant %damage bonus, regardless of how much armor the enemy has see here.
It doesn't matter if your enemy has 300 armor or 15 -- penetrating 10 of that will have the same effect on total DPS increase.
That said, let's take a look at why Armor Penetration is preferred, even with its linearity.
Attack Speed is easy to acquire via items.
Armor Penetration is not.
You can more than match the effects of attack speed runes with a single dagger - 420 gold.
The cheapest item with Armor Penetration is the Brutalizer at 1337 total gold. By this metric alone, Armor penetration runes are worth approximately three times that of attack speed runes.
It is easier to use items to stack Attack Speed over Armor Penetration -- but to get the highest DPS, you want to stack both.
Given that the amount of Armor Penetration available via items is strictly limited, if one's goal is to achieve maximal values for both Armor Penetration and Attack Speed, runes for Armor Penetration are much better than that of Attack Speed.
Best Answer
Personally I think getting a second rabadons is a bad choice. There are other items that give a lot of AP (close to 120) plus you get lots of other stuff as well (for example Deathfire Grasp which gives you 100 AP and 15% cooldown reduction and a great active which is like one extra spell). The reason rabadons is so great is not the 120 ap. It's the passive.
So in my opinion you don't have to get into the whole magic resistance vs magic penetration calculation.
If your dilemma is Second Rabadons vs Void Staff - Just go Void Staff.
But if you want a truly damage based comparison overlooking every other aspect of the game lets do some math using this website: http://idunnolol.com/lol/calculator.html
If you have
You will get
with void staff you got 35% magic pen so you get 65 magic resist and 70 ap so you have
and you get
So its even when the opponent has 100 magic resist. If he has more magic resist void staff beats rabadons if he has less rabadons beats void staff
also if your spell has more Ap Ratio than 1.8 Rabadons wins if your spell has less Ap Ratio Void Staff Wins.
You can do the math for any spell you want. But if you dont need void staff then I still insist you get something else instead of Rabadons.