Okay folks... calling it now... the Sole Survivor, is an android (or gynoid where applicable).

possibility 1 - Vault 111 is not a real vault destined to save people, but a facility for testing humanoid robots by periodically releasing them into the world. Complete with built-in memories.

Possibility 2 - escaped Institute android gave himself a new mind and face based on a long-dead vault dweller from before the war.

Because cryogenics would just be too obvious.