You need VCB installed on the Media server, this is VMware software, not Symantec's.  VCB or Vmware Consolidate Backup does most of the heavy work, with BE calling it to do the work.

You should put agents in the virtual guests, but note still do the backups via VCB where applicable.  Application level backups would be done with the agents e.g. SQL, Exchange, AD, database level apps, VSS integrated apps, etc)

There is a trick to the credentials that also have to be edited in vcenter as well.  Refer to the number of documents that already exist in the Administrators Guide, and the support KB website.

so what's the point in even buying the VMWare agent with Symantec if I have to buy the VCB which will do everything for me?

VCB in 3.x is a framework, it cant be used alone.  You have to script everything out by hand if you use VCB alone.  Go ahead and use VCB and manage it with scripts.  Good luck ;-)

4.x got rid of VCB in favor of vSPhere.

The virtual agent does more for recoveries than it does for backup anyways.