I have multiple instances of nova-compute and have had to edit nova.conf on each host to make vnc work. The relevant options are:

novncproxy_base_url = "http://10.10.5.4:6080/vnc_auto.html"
vnc_enabled = true
vncserver_listen = "0.0.0.0"
vncserver_proxyclient_address="10.10.5.13"

Is there a way to put multiple IPs in the juju config (vncserver_proxyclient_address)so that it does not keep getting overwritten.