From your above post it is very clear , you have some issue with VSS , but it is not clear during the backup or restore
Please post with more information when are you receving the VSS error
Have you ensured on server by typing vssadmin list writers that al writers are stable
Easiest way around this is to reregister your VSS *.dlls first and see if this fixes the issue. Check the TN below for Win2K8. I've also used it for Win2K8 R2...
If this is Server 2003, then do a Google search for Windows Server 2003 VSS Rollup Patch and install it. This requires a restart.
For your disk space, you can check the TN below to see if it applies or not:
Sometimes a server restart also helps, so you might want to consider that too.
...if it is failing your backups, your job log should either state the server/s it is failing on, or give you an error message. You'd obviously run the vssadmin command on the server giving you issues, or post the error message here...