"Your key vault and VMs must be in the same subscription. Also, to ensure that encryption secrets don't cross regional boundaries, Azure Disk Encryption requires the Key Vault and the VMs to be co-located in the same region." https://docs.microsoft.com/en-us/azure/virtual-machines/windows/disk-encryption-key-vault

In the Reviewers section, select either one or more people to review all the users in scope. Or you can select to have the members review their own access. If the resource is a group, you can ask the group owners to review.

Reference:
https://docs.microsoft.com/en-us/azure/active-directory/governance/create-access-review
https://docs.microsoft.com/en-us/azure/active-directory/governance/manage-programs-controls

Reference:
https://docs.microsoft.com/en-us/azure/active-directory/roles/delegate-by-task