To be fair, its stated by one of the Children of the Cathedral that there god had decided to take a "Less Mobile Form" suggesting that before he attached himself to the LA Vault Computer, he probably could have very well slithered along the ground. Is it so hard to believe that the Master just sludged across the floor? Because there was an experimental Vault in LA that he thought may have at some point had residents. Plus it is in close proximity to Vault 12, and The Hub, so he could easily keep his eye over there. Opposed to Mariposa which is in the middle of nowhere. I believe its mentioned that Morpheus formed a doomsday cult after leaving The Rippers. If your waiting for the end of the world, and then hear about a huge telepathic blob that eats people alive, of course you are going to worship it. Is it too far fetched to suggest that he found them. After all, he needs spies/technicians/propaganda ministers, and why not get a bunch of loyal cultists to do your dirty work? To be fair, only caravan merchants actually trade between The Hub and Boneyard, and they likely wouldn't have encountered them themselves(Else they'd be dead). So they've probably just heard rumors of them, and they slowly became boogeymen in other parts, that believed they were Devils/Ghosts/Vampires/ECT., henceforth the legend of the Deathclaw begins.