[Yaffs] VFS and/or YAFFS Direct

I'm am working on adding YAFFS to my OS's VFS. I've looked at
yaffs_vfs_single.c that's included with the YAFFS distribution and shows
how YAFFS was added to the Linux VFS.

It seems to me that yaffs_vfs_single.c must implement things that are
implemented in yaffsfs.c (YAFFS Direct). So, is there a reason that a
YAFFS VFS implementation can't make calls into YAFFS Direct?