Complex sentence: Understanding that all life on Earth-Vault and Non-Vault-were minorly irradiated, the Enclave modified FEV to destroy all these "mutants"-thus allowing "humans" to resettle.
Split into simpler sentences: Understanding that all life on Earth-Vault and Non-Vault-were minorly radiated. The Enclave, seeing that these "mutants" existed, modified FEV to destroy all "mutants"-thus allowing "humans" to resettle.