Limit the VMs access to the Cloud Storage buckets by setting the relevant access scope of the VM.

Create IAM bindings for the VM’s service account and the required buckets that allow appropriate access to the data stored in the buckets.

Grant the VM's service account access to the required buckets by using domain-wide delegation.

Create a group and assign IAM bindings to the group for each bucket that the application needs to access. Assign the VM's service account to the group.