To limit the search to particular vCenter or ESX servers

NBU for VMware Admin Guide  http://www.symantec.com/docs/DOC3663

Extract from p. 72:

The Virtual Machine Intelligent Policy feature in NetBackup 7.1 is a new approach
to virtual machine selection in the policy. It represents a paradigm shift in the
way you select virtual machines for backup. As with all major changes, the effective
use of this feature requires forethought, preparation, and care.

See table on p.73: Important notes on automatic virtual machine selection!

Have a look at chapter 6 in this guide : Configure automatic selection of virtual machines for backup

Also see how to Limit access to the datastore globally (across all policies): Use the Host
Properties Resource Limit screen.
See “Setting global limits on the use of VMware resources” on page 40.

Hope this helps!