Every Ninja Trick that mimics a spell says in the description that you use your Ninja level as your caster level, but this trick does not say that. So do you treat the greater invisibility as though your CL is equal to your Ninja level? Was this an oversight in the writing or intended?
You don't need a CL here. CL only factors into greater invisibility in duration, which is here inherited from vanishing trick instead, and dispelability, which is irrelevant here as supernatural abilities cannot be dispelled.