Vault is not suppose to be something that you keep gaining money, it is suppose to be something "if you are a gambler, and you are not currently gambling, you can put it in here" type of deal
Its kinda similar to rakeback that is very common in most sites but the the amount of the 'rakeback' you will get from the vault will be depending on the amount of The Vault and your share of that vault ( percentage of chips you owned ).
I cant fully agree if you say the vault in chips is similar to rakeback in most other sites as it is completely different.
Rakeback is something fixed, calculated based on the rakeback percentage and wagered amount by players while vault in chips is not fixed as the amount will always depending on the vault balance.
Vault may give someone bigger amount than rakeback when the vault balance is big but it may less amount than rakeback when the vault balance is small or even negative.