Vault 101 is the initial starting point of Fallout 3, it is home to an unknown number of citizens (at least over 30). The Vault was supposed to never open, but the Overseer and some of the older citizens know about its actual opening in 2258. It was then that James and the baby 'Lone Wanderer' entered.
Vault 101 does not give you a very good safe haven, as the only way to enter the Vault freely is to evacuate everyone in the 'Trobule on the Homefront' quest. Even when evacuated, radroaches spawn in the entrance. While not much of a threat, they are an annoyance. More suitable areas like the nearby Megaton would be recommended.
If Dogmeat dies, the perk "Puppies!" (Broken Steel-only) will have a new Dogmeat puppy waiting right outside of Vault 101.