The veep's office explains why it is not, in fact, subject to mandatory security inspections for offices within the executive branch:
A frequent critic of the Bush administration, [Rep. Henry] Waxman also asked Cheney how the vice president's office could claim, as it has in correspondence he cited in his letter, that it was not "an entity within the executive branch."
One Cheney staffer familiar with the matter said Thursday that the vice president has not complied with the order because his office has dual functions: It is part of the executive branch — the Bush administration — but also part of the legislative branch, given Cheney's position as president of the Senate.
As such, the vice president's office has no legal obligation to abide by the order because it only applies to the executive branch, said the Cheney staffer, who was not authorized to publicly discuss the inner workings of the office and requested anonymity.
Got that? The office is part of the legislative branch when the Information Security Office is performing executive inspections, but–lo!–will shapeshift into executive branch mode come time to invoke executive privilege.
Last year, I explored the link between Cheney's aversion to disclosure and his taxpayer-funded (man) hunting expeditions.