Does pierce x work against units with armor? For example, Darth Vader attacks an AAT. With Vader's Pierce 3, would that cancel up to 3 blocks from the AAT?

Nope. Armor doesn't add blocks, it cancels die results which is why all(most?) Lightsabers come with Impact.

So hope you got as many hits as you have Impact, then rolled the rest as crits :p

Edit: from the rules reference.
ARMOR (UNIT KEYWORD) During the “Modify Attack Dice” step of an attack, if the defender has the armor keyword, that unit can cancel all hit (󲉠) results produced by the attack roll.
• The attacker resolves abilities during the “Modify Attack Dice” step of an attack before the defender resolves abilities during that step. As such, the impact keyword can be used to change hit (󲉠) results to critical (󲉡) results before the armor keyword can be used to cancel hit (󲉠) results.
• When a player cancels a hit (󲉠) result, that die is removed from the attack pool.

36 minutes ago, RejjeN said:

Nope. Armor doesn't add blocks, it cancels die results which is why all(most?) Lightsabers come with Impact.

True, armor doesn't add block results, but units with armor still a need to roll saves against crits, which pierce DOES work against.

In the Vader example, the steps would be:

-Attacker rolls dice

-Attacker converts up to 3 hits to crits due to Impact.

-Defender cancels all remaining hits due to Armor

-Defender rolls dice equal to remaining crits

-Attacker cancels up to 3 block results due to Pierce

Fair enough, I mistook the question to read: "Does Pierce ignore Armor."

Just to be clear to the OP, the answer is a clear and simple "yes."

On 8/4/2020 at 6:21 AM, Lochlan said:

True, armor doesn't add block results, but units with armor still a need to roll saves against crits, which pierce DOES work against.

In the Vader example, the steps would be:

-Attacker rolls dice

-Attacker converts up to 3 hits to crits due to Impact.

-Defender cancels all remaining hits due to Armor

-Defender rolls dice equal to remaining crits

-Attacker cancels up to 3 block results due to Pierce

you are right for a melee attack but if they are attacking with saber throw then you would apply cover before converting for impact,