Rob,
No you are not required to only have one backup client in a VCB policy, but you may find it helps. We have no more than 3 VM's per policy, as we are willing to take a chance on only having to rerun the backups for three clients at a time. We are waiting to upgrade to 6.5.1 to get the full benefit of the VCB integration with NBU.
VCB is going to create its snapshot files for each server on your VCB proxy/NBU media server one after the other, not all at the same time. Then they will get backed up, again, one after the other from the proxy to tape. If you have more than one client in the policy and a subsequent snapshot of a client fails, or the backup of that policy fails, all of the snapshots will automatically (usually) be deleted from the proxy and rerun from the beginning. This can cause what should have been a short backup process to take twice as long, assuming you don't have further problems. That cleanup isn't always foolproof, though.
As for only backing up selected VM's, absolutely, you can do that. The actual client name you will back up will be the name of the proxy server, not the individual host names. Then, in the Backup Selection List, you will be specifying which servers to back up in that policy in a special format, like this: "f:\mnt\servername.mycompany.com-FullVM". If you are backing up more than one VM in this policy, you will have more than one line like this.
FYI, we have also encountered times where a backup fails, cleans up all of the snapshot files on the proxy, butVCB thinks a snapshot for a client still exists, when it does not. We then have to have the VMware team do some magic on their side to reset it before we could run the backup again.
