Not sure why you are using pre and post commands? If you are doing because you are not licensed for the Vmware Virtual Agent, then that makes sense. The Virtual agent eliminates all scripting PERIOD. All jobs are managed like normal, browse to your virtual center/ESX hosts, and check a box or multiple boxes/servers for your selection list.
For VCB with scripting, you need to probably refer to the Vmware forums, as they are better at those things then here. They will help you specifically with the scripting needed, which you will enter in BE. We can help you here for sure, with any BE specific issues.
I seem to recall some BackupExec 11d scripts from the VCB installer or available for download from Vmware that you can tweak yourself. See if you can get those?