Assuming that was a typo for --vss-use-mapping=true. Verified that it fixes the VHD test case.
Activate this option to map VSS snapshots to a drive (similar to SUBST, using Win32 DefineDosDevice). This will create temporary drives that are then used to access the contents of a snapshot. This workaround can speed up file access on Windows XP.
which sounded so inapplicable that I forgot it existed, but now it’s nice. Thanks for the suggestion.