The above are true. The vault has been unsealed before. Presumably it was unsealed when James was in town but was about to go into lockdown.
Each overseer gets to choose whether to keep the vault open or shut, regardless of what their original orders were, no one can stop them from deciding.
The previous overseer had the vault open for a long time http://fallout.wikia.com/wiki/Unnamed_Overseer
The Vault suit Moria gives you was likely from one of the previous vault "ambassadors" as "weapon with a skirt" calls them - possibly Anne Palmer
So saith I, SentientSurfer.