You seem to have answered most of what you are asking.
If what you downloaded is a ZIP file, then it was mis-named to be an ISO.
Use UNZIP to extract the ISO file and then burn it as an image.
The file is not really an ISO file, it is a bootable disk image in ODS-2 format.
Try a free tool like www.imgburn.com or cd-record to burn the CD-ROM. If visiting the imgburn site, be aware that the real download links are in the version announcement, the big green download buttons appear to download an ADWARE loaded program that may also download imgburn.
malmberg December 12 2017 HPE only makes the most current version of OpenVMS Alpha / IA64 /VAX available to hobbyists. When I had access to the Alliance 1 program it was the same. No public downloads are allowed by HPE.
nmbonao December 08 2017 Is anybody knows OpenVMS 8.2 version downloadable version? or CD copy? Thank you very much
aarommes December 02 2017 Bitcoin and Blockchain enthusiasts ( plus distributed computing ) please connec / reply: http://www.openvmshobbyis t.com/forum/viewthread.ph p?forum_id=130&thread_id= 2991
malmberg September 10 2017 https://sourceforge.net/p /vms-ports/wiki/VMSInstal lation/ For the most part just use VMS 6.1 media instead of 7.3. But why run the older release?
DoeveR August 07 2017 Where can I find the write up on running VMS 6.1 using the simh emulator?
Bart March 20 2017 Happy to have found my password again!