may just be reading to deep into this, but i noticed that right when you get your pip boy you have some grenades, a sniper, and a laser musket. before in the character creation the wife says something like "you'll knock 'em dead at the veteran's hall tonight." Maybe that's why you get a spot in the vault, because your in the military?
of course, those weapons could just be there to show off those fancy-pants new 3d models for items in the pip-boy.