You could try to go with another route. Login to Cloud Director with vCenter credentials. For this, you can configure identity providers (LDAP or use vSphere SSO as SAML provider). More on that in kb: https://docs.vmware.com/en/VMware-Cloud-Director/9.7/com.vmware.vcloud.admin.doc/GUID-67F52905-87CC-4EF2-94E1-9512A681025A.html
Sorry, if this solution do not work for you. I think Cloud Director is higher in hierarchy, and simply does not speak to underlying vCenter SSO, to push new credentials. I couldn't find any information about your use case. I also think, that you should perceive dedicated vCenter Server as separate entity, as vCloud Director is only a CPoM for it, an a http proxy, just for a convenience case.