That said, there was a character in Fallout 3 that was supposed to have come all the way from the west with the Enclave (Brandice, in Greyditch), so cross-country travel isn't impossible.
although the Enclave did have the advantage of Virtibirds which must have been semi long-range vehicles. And the BOS walked from the West too, via the Pitt. Quite a trip.
James isn't a bookworm though, if you travel with him he can hold is own with a gun. So he was born outside of a vault, a wastelander, but in some kind of scientific college/tribe setting.
I always wondered whether they weren't Enclave. Working on Purity expressly for the purpose the Enclave had in mind, although may be have been working under "need to know". And when the scientists found out that it wasn't actually to give fresh water to the wasteland, they did a runner. When the LW was born, James got into the vault "for safety"... from whom? Why didn't he just hold up in Rivet City with Lee and the rest? Because he knew how to make purity work, so he was the one the Enclave were hunting.
Maybe. Or something.