I've always thought he said "all inquiries made," although it is hard to tell.

The "no inquiries" explanations make sense, but only up to a point.

My problem with them is this: Is it really believable that someone like Vito Corleone would simply shrug his shoulders and let the killing of his son go without even the most routine investigation? If I was Tattaglia or Barzini, I'd be very suspicious if there wasn't even a hint of inquiry from the Corleones.

Also, inquiries and a Commission meeting are not mutually exclusive. Vito could be playing both ends - calling a meeting to arrange a truce, but also being prepared to fight should those efforts fail. In fact, if I was a head of a Family, that's just what I'd expect Vito to do.