“Well the Merchants from the Hub told us a bunch of caravans disappeared on their way up north. I think there's an army in the mountains, but the Elders, well, they don't want to act until they're sure.”— John Maxson
Go to Mariposa Military Base to become aware of the mutants inside the facility, then exit. If the Vault Dweller wants help at Mariposa, skip to the Brotherhood. If the Vault Dweller wants help at the Cathedral, they must then go to the Cathedral, enter the hall and go upstairs to the fourth floor. Deal with Morpheus and hack his computer. Afterward, proceed down to the basement area and uncover the secret passage leading to the Los Angeles Vault.
After this had been done, return to Lost Hills. Report to Maxson and convince the High Elders about the danger from the base. If the Vault Dweller was at the Cathedral and mentions the controls from it, they will send assistance at their discretion, either to the base or to the Cathedral. Otherwise, the Vault Dweller can only take help to Mariposa.