Not possible with NBU 7.1.x - you need to be on NetBackup 7.5.0.3 or later.
Please see Statement of Support for NetBackup 7.x in a Virtual Environment: http://www.symantec.com/docs/TECH127089
Look for these topics in the doc:
Supported platforms for the VMware backup host and backup media server
NetBackup appliance as backup host
Oh! One more thing - NBU 7.5.0.4 supports backup and recovery of vSphere 5.1. (in same doc referred to above)
So, you really need to upgrade. I recommend that you upgrade to 7.5.0.6.
