OpenVMS (also called VMS, and previously called VAX/VMS) is a proprietary operating system with a very long history. It was initially released in 1977 by Digital Equipment Corporation for their VAX minicomputers. Today VMS continues to be updated, and is used to run some of the high end servers of large organizations, including banks, hospitals and industrial manufacturers.
NetHack has continued supporting VMS. However, recent versions of NetHack for VMS have only been tested on Alpha and Integrity/Itanium/IA64 processors. The DevTeam doesn't know if NetHack 3.6.5 can run on VAX machines.