The vault is an additional feature in the game, unlocked when you first earn Keys by ranking 15 or above in Legend Tournament. In the vault you can upgrade some quality of life upgrades and some additional enhancements to your tower and its strengths.
The vault consists of two Tech Trees: the Harmony Tree and the Power Tree that offers unique upgrades that require Keys to be unlocked. The Power Tree is focused towards Tower-related buffs, whereas the Harmony Tree is primarily focused on non-tower related buffs and qualify-of-life upgrades.