the arguement isn't about getting another vamp scepter.
Go read the section that talks about utility in life steal in your link.
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its about your 3rd item.
normally people get berserker grieves, ifinity edge and phantom dancer. then the debate is, go for last whisper or blood thirster. from that link, they're argueing if you run ad runes that last whisper is better cuz you need 10 stacks on your bloodthirster to out put the same damage as last whisper, meanwhile more than likely everyone besides the designated tank won't have that much armor.
10 stacks on BT is nothing, and generally no one aside from a tank/bruiser will have any armor, so BT will generally do more damage.
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so with 40% armor pen allows you to deal more damage early in the game instead of late game.
It depends on the other teams comp and builds.
Yes, if you have full AD in runes it scales like they said. But if you have any armor pen in runes it throws the defense needed for LW to be better than BT a lot higher since your flat armor pen devalues LW defense pen %.
Also if you build brutalizer is throws it off even more (obviously a lot more selective to champs here).
So, it basically comes down to life steal utility, your own flat armor pen, and their team set up/build.
LW comes more in to play when people start building their last defense item in most games/builds.