This is just my 2 cents worth.
I don't really know anything of the VFS code, but would it be easier to
leave the old VFS code alone and write the new VFS code as a sort of "shadow
system" which does not really give rise to disk operations, but just
compares its state with that of the old VFS system and panics (or does
something else noticeable) when it is inconsistent with the state of the old
VFS system? Afterwards when the new code is done you can rip out the old VFS
code in one blow.
Jeroen