What is Vault?

In addition to the regular rewards you earn from beating levels, you also receive a handful of extra Coins. These Coins are stored in a special safe – the Vault – you can see at the top of the screen. Once the Vault is full enough, you can open it up and get all the Coins inside at a special discount.

If you don't empty your Vault, it will eventually fill up all the way. Once that happens, you'll stop earning extra Coins after each level, but you'll still get your regular rewards as normal. Redeeming the Coins in the Vault will empty it, allowing you to gain extra Coins again.