It wouldn't make any sense to go to Los Angeles or San Francisco. The distance between Las Vegas and those two cities are quite far. LA is 265 miles away and SF is 564 miles away. It would be absurd to go to either one. Area 51 is 126 miles away from Las Vegas. It could be plausible for them to go there, but hopefully not.
That's what the Vault Dweller/Chosen One traveled in the originals, it just took a realistic amount of time with the node system. It fits more within the scope of the base game to go within 2 weeks of walking from the section of the Mojave we play in, but they might be able to come up with a good reason for Los Angeles.
Also, to those who are now speculating on the "personality from Fallout 3" from that "reliable source close to Bethesda", it is old news, and unreliable old news. Seriously. It makes little sense for people to come over from Fallout 3 (across the country), and if they found a reason, it is hard to make a good one. All evidence points to there being 4 DLCs, the thing with the source saying Ulysses is the Couriers lover is BS, and the source is completely dubious. I was happy when that ridiculous "source" was forgotten.