You’ve got the cistern (bottom left) which is cabal. Glade of Echos and Exodus black which are fallen (bottom right and right)
You might be thinking of Artifact’s edge which is the center area but all it’s got is a cabal PE and some vex wandering around.
It’s just a storage area you can take it out and use it whenever you want. The only benefit to having it in the vault is that it will raise all your characters base power level so if it’s a high level weapon that you don’t like you don’t have to swap it between characters to get higher level drops.
I don’t but you should use what you like.
It unlocks the next subclass. There are 3 in total for each class. For hunter it’s Arcstrider, Gunslinger, nightstalker.