It seems to me that you're conflating two assertions: 1) the original, "the US was behind the 9/11 attacks", and 2) your offered alternative, "the VP and a handful of others had specific, actionable information on al Qaeda's attack and did nothing to stop it". The two are not the same, and the preceding comments were made regarding the first.
In either case discounting a conspiracy as impossible isn't silly if there's no evidence of said conspiracy nor even a plausible hypothesis of how such a conspiracy could have worked. The onus of proof is on those proposing a conspiracy, not on everyone else to prove a conspiracy didn't occur.
In either case discounting a conspiracy as impossible isn't silly if there's no evidence of said conspiracy nor even a plausible hypothesis of how such a conspiracy could have worked. The onus of proof is on those proposing a conspiracy, not on everyone else to prove a conspiracy didn't occur.