Microsofts Key Vault tech support team has confirmed that the Azure Resource Manager (ARM) outage took down Azure Key Vault as well.
A detailed post-incident review (with tracking ID NKRF-1TG) is currently available on https://azure.status.microsoft/en-us/status/history/.
We expect that the collective set of mitigations applied by Microsoft will be effective in significantly reducing the risk for this type of incident happening again.