Starting life as one of the very first 24-Hour RPGs, the original vs. Monsters was created over a wild 24-hour period. Since that time the system has grown, expanding into a commercial version of vs. Monsters and then into an OGL system that any fan or publisher can use to create their own games.
To use the vsM Engine you will need either the SRD or a vsM Engine game and a deck of playing cards.
Download the vsM SRD and the engine at http://www.roninarts.com/store/default.php?cPath=5_14
PH - note as of 21-FEB-2022 the above link does not work.
There are files at the following dropbox location - please take the usual precations when downloading files of unknown origin.
vs. Monsters is available from Lulu.com