True. The enclave were not in Fallout 1 but they are now a very big player in the Fallout universe so i find it difficult to not imagine there involvement in future games.
Why? They were pretty much destroyed in Fallout 2, before Beth decided to bring them back. Black Isle themselves didn't plan on reusing them.
I just feel the pipboy has to be aquired from a technology source such as a vault which has no obvious military agenda.
How about the Commonwealth? They're high-tech, but not necessarily militaristic.
Anyway, the Pip-Boy was created by RobCo before the War, and the vaults were probably not the only ones to use them. There might be some wastelanders who simply stumbled upon a RobCo factory. The librarian in the Hub in FO1 recognized your Pip-Boy instantly, so they can't be totally unheard of (even if rare) among non-Vault people.