You will want two elevator shafts eventually as your vault builds up, to provide maximum mobility.
Elevator – Chained together to make elevator shafts.
Staff your two most butch fighters with their best weapons here to guard against invading raiders.
Vault door – You get exactly one, always on the top floor at ground level.
Utility rooms – Required for vault function. Rooms can be categorized by function, thusly: Most rooms can be expanded to 3 sections wide and later upgraded. Be careful not to overbuild, because rooms left empty tend to attract disasters. Over time you will need a good balance of rooms to provide stable function levels. The rooms are what your vault is made of, so plan them carefully.