As its a vSphere limitation, Backup Exec currently not supporting the backup of VVols through SAN transport mode.
Please check below links for more information.
"vSphere Virtual Volumes does not support SAN transport mode. vSphere APIs - Data Protection automatically selects an alternative data transfer method."
"For today’s VMFS disks, SAN transport mode relies on a proxy VM to tell the backup appliance which blocks it should read (since the Windows OS on the backup system can’t directly mount a VMFS disk). With VVols we use the VASA API to establish a “binding”, something of a “handshake” between the ESX host and the VASA provider, which not only instructs the VASA provider to “construct” a data path for the VVol for the ESX host, but also provides the requisite information needed by the ESX host to construct the data path on the ESX side. Because physical backup proxies cannot be VASA clients, currently, it is impossible to construct a “direct” data path between the VVol storage and the Physical Backup Proxy.”