In the key vault portal, when you view a certificate, the top URL is the "Certificate Identifier". If you specify this in Express V2 deployment, it will only contain the public key. If you need the cert with private key, you have to go to the bottom of the page and look for the "Secret Identifier" URL and use that instead. IMO, the "Key Identifier", "Secret Identifier" and "Certificate Identifier" URLs should all be at the top of the certificate version page, and they should have some help text to indicate the difference.