I used to use the vault for my Teen celebrity, to keep his earnings separate from the family household funds. There is an interesting side benefit. Sims can woohoo in the money, if they're so inclined. The vault can be cumbersome, though and it isn't beyond being hacked. Twice, Erik's siblings launched an attack on it. Once his next younger brother, who is a Genius, and then his younger and only sister. Bree, how could you? I was very happy to come across SimRealist who has mods that allow for Sims in the same lot to have their own bank accounts. I found I liked this better, so I wrote down how much Erik had accumulated, transferred the funds to the household and from there he banked it. Then I removed the vault. As for the door, I think you have to build a room onto the lot and then use the doors to gain access. I've never had any luck using it.