I've seen similar with RHEL7 guests in vSphere 6, with the following notes and workarounds:
-Installing a 'minimalist' version of RHEL7 and then installing open-vm-tools (from the RHEL7 DVD yum repo) as well as the deployPkg Tools Plug-in (from VMware following KB2075048), guest customization doesn't work properly. It appears to run but the resulting clone is not unique.
-Installing RHEL7 along with the Guest Agents Add-On, open-vm-tools is automatically installed (from the RHEL7 DVD) as well as the deployPkg Tools Plug-in (from VMware following KB2075048), as well as some maybe other package(s) bundled with Guest Agents whereby guest customization and resulting clones are unique.
The difference between failure and success points to the delta between manually installing the open-vm-tools package from RHEL7 which is failure prone and deploying RHEL7 in the Infrastructure Server role along with the Guest Agents Add-On which works.
(In either method, for guest customization to have a chance to work, you must install the deployPkg Tools Plug-in from VMware following KB2075048 before sealing the template and attempting customization or cloning)
There may be more to it than that but I'm still researching in the lab via trial and error.
EDIT: I've got it sorted. I updated my findings and many other personal tidbits in the following blog post. A RHEL 7 Minimal Install is missing PERL which guest customization requires. The installation of deployPkg Tools Plug-in from VMware doesn't check for the existence of PERL.
http://www.boche.net/blog/index.php/2015/08/09/rhel-7-open-vm-tools-and-guest-customization/