Are you able to access the web console of virtual appliance 5.1 https://<vcvaip>:5480 ? first you have to configure SSO and virtual center database via the web consloe or cmd .Make sure all services(vpxd,sso,is and vsphere client) should be running.Once you are done with these configuration on your virtual appliance then you can access virtual center server using vsphere web client ? https://<vcvaip>:9443 or vsphere client.
Thanks,
Atul