So... almost all the floppy drive image formats used for PC-9801 games are essentially uncompressed and unencrypted... except VFL. Also, almost all are supported by VFIC to some extent... except VFL.
The utility is fortunately still around:
https://www.vector.co.jp/soft/dos/util/se012084.htmlSadly, although the documentation hints at a complete file format spec (separately, on NIFTY) I haven't had any luck locating it. The author states the images are compressed using an algorithm borrowed from LHA. Together with being able to use the utility to make test files from known uncompressed images, it's probably quite possible to reverse engineer it.
My question, though, is why? I've only ever seen two games in VFL format:
J. League Professional Soccer 1993
PlayMaker Football
Has anyone seen any more? Do any emulators support this format?