Hi,
I think TSM does not support backup for virtual machines that have either independent disks (either persistent or non-persistent), or that have raw device mapping (RDM) disks configured in physical compatibility mode. RDM disk configured in virtual compatibility mode are supported.
Hence I would suggest you to take the LUN snapshot or mirroring at the storage level as a backup.
That is make a flash copy of the production LUN --- > Disconnect the relationship between two LUN after 100% completion --> Take the tape backup of that secondary LUN.