In Mobile Strike the Vault allows you to deposit gold for either 1 day, 3 days or 7 days. Once your deposit matures you will receive your original gold back, plus interest in the form of extra gold!
Upgrading the vault allows you to increase the amount of gold you can invest and the interest rate, offering you greater returns.
Unlike the other buildings in Mobile Strike, the vault can only be unlocked and upgraded by purchasing gold packs. To unlock the vault you can purchase any pack (including the cheapest). To upgrade each level requires you to buy packs with a certain amount of gold e.g. level 2 requires you to buy a pack with at least 8,000 gold. Later levels require you to buy packs for a certain dollar price.
The daily returns actually work out better the shorter you invest for, but you have to remember to keep reinvesting. i.e. over 7 days you are better off doing seven 1 day investments, than a single 7 day investment.
Level 5 deposit at 1 day works out at 990 gold a day or 29,700 gold a month.
Whilst invested your gold is completely safe and can be withdrawn at any time (although you will lose any interest if you do).
The table below has the build cost for each upgrade level and the rewards granted for your Vault.
Vault Building Requirements
At level 5 you also unlock a 14 day deposit which allows you to deposit 12,100 gold. The starting rate is 20% and the bonus is 34% giving you a 54% (6,534 gold) return on 14 days.