Failing backups when quiescing is used with Vzshadow installed on 2008 VM's.
Vzshadow is looking for a specific dll within the C++ redistributable libraries and can not find this DLL or vzshadow is in the incorrect location.
Check the VM's event viewer for any errors that may have occurred during the quiescing of the VM. If you see an error stating a dll could not be found, check that the 2008 C++ redistributable is installed on the VM.
Also make sure your pre-freeze script is located in C:\Program Files\VMware\VMwaretools\backupscripts.d\.