I was integrating Citrix XenDesktop 7.5 with my lab vSphere 5.5, and at the point I was configuring my first XenDesktop site XenDesktop throws the error "Cannot connect to the vCenter server due to a certificate error. Make sure that appropriate certificates are installed on the vCenter server" when I point it to my vCenter Server 5.5. This halted the configuration of XenDesktop 7.5, whenever I had installed XenDesktop 7.5 before in my lab I had used Citrix XenServer.
I browsed to C:\ProgramData\VMware\VMware VirtualCenter\SSL and copied the cacert.pem file to the NETLOGON file share on one of my Domain Controllers.
I then imported the cacert.pem file into the Trusted Root Certification Authorities\Certificates store on my XenDesktop 7.5 Delivery Controller. You must use the All Files drop down when you point to the cacert.pem file from the Certificate Import Wizard.
When the vCenter certificate appears in the Trusted Root Certification Authorities\Certificates store, you can retry the XenDesktop site creation wizard. I have found you have to reload the Citrix Studio in order for it to work.
You should probably be looking to use trusted certificates in a production environment, but as this is only a lab self-signed will do for now.