Do I spend Vault keys now or do I wait for Vault event?

[Shimrra]

The Vault only increases the odds of Gnomes and dropping Vault Keys. Use 'em whenever you want.

@Sirrian

The Vault

During a Vault event there is an increased chance to encounter a Treasure Gnome (see below) in a battle
The Vault also increases the chance of Treasure Gnomes dropping Vault keys allowing access to the Gnome Vault.