This absolute beauty of a rear derailleur was intended for my super-rare Victor Vincente of America Topanga refurb project… which never saw completion before parting with it 🙁
Yes, the Suntour VX-S is the rear derailleur spec’d in the build list for the Topanga, the first production mountain bike in history.
Suntour road/touring derailleurs were the obvious choice for early US mountain bike pioneers, due to being robust and reliable enough to handle offroad riding.