Verified with POST /api/vms/:id { "action": "edit", "custom_1": "new_custom_identifier", "description": "new_description" } that it's now possible to edit vms resources using REST API.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHSA-2017:1367