I did do a post somewhere speculating on Aircraft in the Fallout Universe... Who has them, what air technology is possible in a post apocolyptic environment easy (and this was built on by several other posters), but I'd be suprised if major powers such as the NCR didnt have some elementary aircraft (even if they are experimental one-offs or even a simple hot air baloon), but nothing in any major number or major strategic use.
Wait, if they weren't finalized until the Enclave, then how come in Operation Anchorage they have working vertiberds?
Operation Anchorage is not a historicalically accurate recreation of the events in Alaska.
Development
Entries found in a terminal in the Outcast Outpost detail the development of the simulation. The entry dated August 15, 2077 describes how General Constantine Chase tested the simulation himself, but claimed it didn't "feel right," and that hardware problems were prevalent. The second entry dated September 6, 2077 tells of how Chase was making elaborate changes to the simulation which the writer of the entry felt were "divorced from reality," and that he was sure that the real Anchorage Reclamation had never happened the way Chase portrayed it. The writer goes on to say that everyone was beginning to think Chase was going insane, but no one confronted him because they feared that the military would take over the project. At the very end, the writer questions why running the simulation was necessary anymore, since "any tactical data was mined months ago".
In particular, the VB-02 Vertibirds were present in the simulation in full operational capacity when in fact they wouldn't even reach prototype stage until much later. There is also doubt over the plausibility of Chimeras as shown in the simulation, since any other source material on the Reclamation makes no mention of it.