Even though the work is done over the SAN and through vSphere NetBackup does actually do a lookup on the client during the first phase of thebackup attempt so host name resolution is important

I am a little surpirse it gives a 156 error - but I guess as the snapshot is the first thing it does then this is most likely the reason

Create a \netbackup\logs\bpfis\ folder on the VMWare Backup host to capture the bpfis process that does the snapshots and it may make it clearer exactly what it is trying to do

I also remember that an IP address as a VM Display name is not supported

You need to check the bpfis log to get more information why this one fails

Post it on here for us to see

Do you have vm tools installed in the vim in question?

Are you typing the display name manually or are you selecting the VMs using the browse option.  If you are typing the display name manually.  It is case sensitive, and thus will cause backups to fail with Status 156.  In the bpfis log, it will show "unable to locate VM".  However if you are browsing for virtual machine and selecting the virtual machine.  Then the bpfis log would be helpful in identifying the problem, but also check your virtual center to verify they are no snapshot errors showing up in the recent task and events.

m selecting VMs using browse option...