Virtual Machines Configured with RDM Physical Compatibility Mode Disks
Backup Exec 15 and the Agent for VMware and Hyper-V cannot protect Raw Device Mapping (RDM) Physical Compatibility Mode disks using image-level backups.
Physical Compatibility Mode RDM disks bypass the vSphere storage infrastructure and the VMFS file system, and cannot have a snapshot taken through vStorage API processes. Physical Compatibility Mode RDM disks in this configuration are skipped automatically during backup job processing. Associated backup jobs are displayed as successful with exceptions.
To fully protect virtual machines configured with Physical Compatibility Mode RDM disks, the Backup Exec Agent for Windows or Agent for Linux must be installed on the virtual machines to protect them using agent-based backups.
Like it's explained in above link, Symantec Backup Exec 15 has that feature by default, where it will do backup with Exceptions. that probably means, it will ignore those physical mode RDMs and include successful backup of only those devices which can be really backed up by it.
While in VDP from Vmware it's mentioned clearly that following disk type backup for full-VM image is not supported.
Independent Mode virtual disk, Virtual Compatibility Mode RDM with Independent Mode or Physical Compatibility RDM
So safest way to take successful backup of VMs with RDM would be agent based backup.