simple he has a zanpaktou that makes you sense whatever he wants you to sense so he could even be having it with Yamamoto whilst killing everybody in the same room and yamamoto would not know a thing. You did see how Shunsui was easily lead into being a witness of Aizen's innocence so how hard can it be to make everyone else think he was innocent. besides even if they did have a trial for the Vaizards, Aizen could have just hypnotized them beforehand and make them attack everyone else making them look definitely guilty.

we have to admit that Aizen's Kyouka Suigetsu really is a power house on it's own....