I liked the concept behind Vault 111. On the one hand, cryogenics is a very solid option for the kind of space travel the Enclave is testing for; on the other, it's an easily justifiable idea for waiting out the apocalypse. No worrying about food stocks or overpopulation, just flash-freeze the suckers and let them rebuild civilisation when they thaw.
I always had it in my head that the 1,000 person limit of a Vault was for the maintenance personnel. Vast stores of cryogenic tubes were for everyone else, at least the ones in urban areas, to be thawed out as needed for specialists or replacement personnel.