In the job created for your vmware backup, go to the vm-ware option, here you have the transport mode options, only select the 'SAN' transport mode. This will launch your backups only through SAN, If that is not available, your job wil fail. If you activate SAN transport at the top of the list and NBD as second selection your backup will try too run through SAN, if this is not possible, traffic will go over LAN but your job will not fail because of possible issues in your SAN connectivity
