If you don't know, you can find a vault-tec logo on a door in DOOMs prologue. You can also find a dead body in hell wearing an iron helmet from Skyrim, but what if..... It meant something more In fallout 4 you can find Nirnroot grown on the Prydwen, this plant is native to various regions in the elder scrolls series. What if fallout took place thousands of years after the elder scrolls and the Great War started after the UAC set up shop on Mars. What if Vault-Tec was partnered with the UAC?The iron helmet could be from a Nord warrior banished to hell? I dunno I think I'm going crazy
Or better theory. The UAC opened more than one portal. They created Vault-Tec to conduct experiments on the inhabitants of the new world. They dropped the bombs as part of those experiments. As for Nirn, whether they have any established presence is unknown. They clearly were interested enough to bring back specimens, and even bring Nirnroot into Boston.