I have a response from tier 2 support.
"As of now the current SOBR design still requires extents to have some space for VBM files.
VBM is written to every extent where we have backup files related to the job.
However, when backup job has backup files only on one extent, and that extent doesn't have free space at all (not even couple of MB for VBM file) then job will fail and won't switch to another extent.
The workaround is to remove a couple of MB so VBM could be written.
When job has backups files on 2 extents, ext1 and ext2, you won't experience any issue: VBM file will be written to both ext1 and ext2, and if ext1 ran out of space completely, then we will fail to write VBM to ext1 but we will be able to write VBM to ext2 and in this scenario job won't fail - it will detect that it could write at least one copy of VBM.
We have already discussed this limitation with RND, and now RND is thinking about ways to improve our workflow/logic here, probably in one of the upcoming patches for v9, but as of now - unfortunately, this is current design, limitation."
Now, we have one Backup Copy job writing to two extents so just confirming if it should be working as suggested in the third paragraph....