It is a good feature obvioulsy. In the past, players used to use a bank account (secondary account) to separate their funds for some reasons. Now some gambling site use "Vault" to give players a better way to separate their funds without sending it to a bank account or withdraw it to their wallet.
I think it associates with taxes of gamblers, mainly associates with taxes, right? Some nations charge taxes on income from cryptocurrencies so the Vault will help them to stay away from problems with taxes, and tax reports annually if they don't withdraw from casinos.
The reasons why people try to bank their money with another account was because they do not want to gamble with all of their money but they also want to make sure they have a certain amount of money to gamble on that session.
For example, let's make the numbers up and say that there is this one dude who will play 100 dollars per day for 1 week, he deposits 700 dollars because he doesn't want to put in 100 dollars every day, there is no need for something like that. What can he do if he wins? Continue to gamble?
He said he will only wager 100 dollars per day, so even if he wins all of it or some of it, there will be some excess, he probably will not lose all of it, so lets assume he has 20 dollars extra, he puts it back to the vault. He does this for 7 days straight and maybe he could be higher than anticipated? That is how it would work with the vault.