VFS itself never needed to be reversed. It's just a library of other binary files that are stored back to back.
It's the IDX file that reads and decodes the VFS and that has most certainly been reversed
IDX is an index file that lists the name and position of each file within the VFS. Once you have that index you just 'seek' the position in the VFS and read in the number of bytes defined in the idx to get the file.