You need only purchase the Backup Exec ESX agent per ESX host not per VM. If you wish to restore or backup the VM host files granularly then you need to buy Backup Exec Agents per VM as well.
Alternatively use VCB to copy snapshot files to a Proxy server and back those up as part of your normal backup regime,
Here is what a buddy of mine recommended. It is priced per ESX host. It's $1695 US and the maintenance is included. I downloaded it today but haven't played with it yet.
