Microsoft Defender for Key Vault should be enabled

Thank you for your question.
In case of Defender for AKV we don't actually do anything with the vaults themselves, so its safe to enable the plan and start taking advantage of the detections provided.

Microsoft Defender for Key Vault - the benefits and features | Microsoft Docs

https://techcommunity.microsoft.com/t5/microsoft-defender-for-cloud/azure-defender-for-key-vault/ba-p/1825055

Worth mentioning that to avoid a potential surprise in the end of a billing cycle we recommend to proactively estimate potential cost using a workbook we created:

Microsoft-Defender-for-Cloud/Workbooks/Microsoft Defender for Key Vault Price Estimation at main · Azure/Microsoft-Defender-for-Cloud · GitHub