Not sure what the VBM file does.
VBM-file is backup metadata file which is responsible for providing information on the backup job, VMs that are stored in the backup, number of backup files/restore points, and so on. It’s also being used during importing backups or mapping of backup jobs to existing backups.Note: Even without .vbm-file import functionality will work fine, though, this process might take a little bit longer.
If I was to backup ONLY the VBK and VBM file to tape, would this be enough to recover the VM in the event of a disaster? Is it necessary to have the VRB files as well?
No, .vbk file will be enough for recovering VM from it, should any disaster happen.
Nevertheless, please be aware that reversed incremental mode isn’t considered as the best practice in case of you copying file to tape, since each job run will produce a new .vbk-file, which is then transferred as whole to the tape. So, it's forward incremental that should be used instead. Furthermore, it might be worth reviewing this
topic regarding tapes and everything which is related to it.
I am looking into different ways where each night I can backup to tape ONLY the most recent version of each job.
It can be done by the means of PowerShell. Recently I’ve done something similar to one of our customers. He was particularly interested in copying only latest .vbk file to remote location in the end of the month, using Robocopy utility to achieve his goals. If need be, I can share it.
Hope this helps.