I dont believe a lance needs to Pierce armour to be classed as effective vs armour,

If you slash at armour with a sword/sabre... Your not going to do much.

But if you charge at the opponent and hit him with a lance.
its going to hurt/knock them down Which makes them vulnerable.... so thats a + to the efectiveness vs armour,

alot depends on the armour though.
A relitivly flat brest plate. even of substantial mass would find it hard to resist a lance that was being propelled on horse back.
its just a matter of PSI...
small pointy thing + horses weight = Lots of PSI.

and if your stupid enough to use a breast plate thats made to resemble a sculptured human body.
Spoiler Alert, click show to read: 
"this pic is a little more extravagant than i was thinking"


A direct blow to the Cleavage, would theoreticaly be enough.

How ever if you have those old french "i think" suits of armour.
That protrude out in the center. and round off to the sides.
Spoiler Alert, click show to read: 
apparantly this breastplate was used at waterloo by a 5ft 9inch brench butter maker. Who lost a fight with a cannon ball.

the lance would not really be able to efectivly strike.

Basically.
for the thing to pierce the armour.
The armour needs to have a flat bit atleast the same size as the tip of the lance.
and you would need to hit it with 100% accuracy.

But even if you dont pierce the armour. The attack will have an Effect. so + 1 effectivenes vs armour,