> cylc vip --help
# States which steps item is valid for
-O OPT_CONF_KEYS, --opt-conf-key=OPT_CONF_KEYS
                        [validate, install] Use optional Rose Config Setting

...
# Does not state which step item is valid for
-s NAME=VALUE, --set=NAME=VALUE
                        Set the value of a Jinja2 template variable in the

# but if we assume it applies to all steps:
cylc install /var/tmp/cylc-src/like.rail --set 'HELLO="hi"'
...
cylc: error: no such option: --set