The way it's supposed to work is as follows;
On the networker server you make one or more client definitions of your VCB server. In addition, you set it up as a storage node for those clients (ie under globals(2) of the client def.)
Furthermore, you configure the backup command for this client def as save_vcb.bat.
Now, in order to actually perform a scheduled backup, you need to populate your client definition with savesets. In the way which VMware has chosen to implement this, you must specify one saveset per Virtual Machine, on the form VM::FULL
Lastly, that client must be made a member of a backup group so that it actually starts on a regular interval.
What will happen then is that when the backup group starts, the Legato module will run VCB once per save set (ie per VM you specified), snapshot and export the disks to the local folder you specified, and back this folder up to tape.
Make sure to read the documentation both for VCB and for the Legato module itself - there is some important info you won't find anywhere else.