You assume that they will accumulate any vast amount of wealth while in the vault. Why would they? It's a vault, they don't even use currency most of the time within their own walls to my knowledge. Other than little knick-knacks that would help with doing miniature quests within the vault for xp and to teach how to use skills on items, maybe some possible functions of them, the vault shouldn't be giving them much when they leave anyway. Nothing, in truth.
Maybe if the idea reaches fruition specifics could be fleshed out and certain players may be able to work within the vault for a few caps before leaving, but it would not be much. Lvl 5 is too high as well, lvl 3 sounds more appropriate. And even then, they might get bored, not even reach lvl 3 but have learned everything and leave early.
Vault is about teaching some basics and hints to those who Want it before being kicked out on their ass, thus the lower lvl cap. People would be bored in there before 5 too I bet. It would also negate most of the need to always tell new players to go to ncr, too. Unless he was a dork, didn't know anything and still chose to skip the tutorial-vault.
