I have Maester Aemon in play, he has Pale Steel Link attached

I lose a Military challenge and kneel Maester Aemon to satisfy the claim effect

PSL allows me to put a 1 cost character or attachment into play when the attached character kneels

I want to be able to bring in Poison Coin at this point - (When attached character stands kill it)

Would the response on PSL trigger at the point the character kneels (claim affect) or does it wait until the response window following the challenge resolution

Ideally I would like to be attaching PC to a character with vigilant and thus need to get it attached before the passive occurs

During challenge resolution, all responses wait until after all the challenge resolution steps AND passives to them. You will not be able to use a response to something that happened during claim until after passives resolve.

Would the fact that the attached character did stand during the FAW be enough to allow you to trigger Poisoned Coin's response, even though it wasn't attached when it did?

Thanks ktom - that's what I thought

I think Khudzlin is correct ... the timing of the attaching or standing should not be relevant to Poison Coin's response ... everything meets the requirements at the time the response is triggered