I'd rather have the boba/plans plot remain anominous so the audience could figure it out for themselves rather than sponfed like lucas's prequels.

I agree with this. You could still have the Fett/Vader conversation, but make it sound like Fett gave the plans to the Bothans, a third party.

Thats' really sinister to have him basically covering his tracks like that.

Maybe have Vader ask him: "Have the plans been delivered to the Bothans?"

Then when Mothma is talking about it the audeince would go "Oh no, a setup!"

Could be the tension we'd need.