I bought 1 vault. Then I saw how limited that storage was on it and how I would have to pay extra upkeep so I just bought another house thru loyalty merchant - those do not charge rent and hold a lot more than any vault. Then I made some alts so they could buy plots so I could more houses up.
You can earn a 500 slot storage box for your house, by completing the "ring of scale master scavenger"-achievement.