During the Vault Event Ricky…
…not the battle inside the vault.
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If anyone get a Vault Key, better keep it until the week of the Vault Event. That’s why having more gnomes is better, you have more chances to get a Vault Key and the sum of all rewards of all gnomes you found could be close to what you could gain within the previous Low Spawn Rate+Better Rewards.
Just consider the gnomes giving lesser rewards (such as minor stones and souls) as if they were battles without gnomes in the intended Spawn Rate and Rewards.