As most folks have outlined its more infrastructure issue than NetBackup.
In addition I'd say your VM's are real busy at time of backup and are getting stunned.
You could run ping tests during a vsphere snapshot event at the VM in question IP address.
You could also turn debugging on for vmware tools for the tools processes to see where it might be getting stuck at.
Look to see if you have any freeze and thaw scripts on the VM's that the tools are interacting with pre and post snapshot.
We experienced a VM that had large amounts of I/O data written constantly drop out as it got stunned.
This issue occurs if the virtual machine generates data faster than the consolidate rate.
Google vmware+snapshot+stun
Might help track down your issue.
From a VMware KB: A snapshot removal can stop a virtual machine for long time
Note: Beginning in ESXi 5.0, the snapshot stun times are logged. Each virtual machine's log file (vmware.log) will contain messages similar to:
2013-03-23T17:40:02.544Z| vcpu-0| Checkpoint_Unstun: vm stopped for 403475568 us
In this example, the virtual machine was stunned for 403475568 microseconds (1 second = 1 million microseconds).
