From my experience this usually is because VC is not able to mount the VMDK to inject the sysprep file. In many of the cases I have seen it is because there is or was another VMware product (such as converter, VCB, workstation, or the disk mount utility) installed on the system and the files for the Virtual Disk Mount Service are the improper version.
To verify if this is correct or not, take a look in the c:\Program Files\Common Files\VMware\Virtual Machine Image Editing folder and look at the versions of the files (usually I look at vmmount.exe first) In general the build should be the same as the VC instance. ie. 2.2.3.40644 would be for VC 2.0.1 patch2.
If the versions do not look to be correct for your version of VC, the easiest way to fix this is to replace the files from a known good VirtualCenter install, or to remove these files and do a repair install of VC and it will put the proper files in here. NOTE: By doing this you will fix VC and will be able to customize VM's, but may break the functionality of the VMware App to be able to mount VMDK's.