Remmina cannot perform VNC remote desktop session before user logs in

I would like to perform a VNC remote desktop session with Remmina between two computers running Ubuntu 14.04 LTS. I am able to do this remote session when a user is already logged into Unity on the remote computer. However, if the computer has just booted and the Unity log in screen is visible (i.e. that a user hasn't logged in yet), I am unable to start the remote desktop session. This forces me to physically go to the remote computer and log in so I can start the remote desktop session.

A solution would be to configure users on the remote computer to automatically log in into Ubuntu, but I would like to avoid this since it is not a secure configuration.

Any ideas on how Remmina could start a VNC remote session even if no users are logged in on the remote machine ?

Remmina is the viewer; Vino is probably the server that you're using, Among other drawbacks, it is unable to run outside of a user session.

X11VNC is what you want. I find that the performance is also much better than Vino; for years, every time I try Vino, I always end up installing X11VNC.

There is a bit of config to be done to make it work right, but here are some skeleton notes on what I did with my last X11VNC installation:

Vino performance still stinks; installed X11VNC
    $ x11vnc -storepasswd
    /etc/init/x11vnc.conf:
        start on login-session-start
        script
        x11vnc -display :0 -auth /var/run/lightdm/root/:0 -loop -forever -safer -shared -ultrafilexfer -bg -o /var/log/x11vnc.log -rfbauth /home/charles/.vnc/passwd -rfbport 5900 -http -users charles -nowireframe
        end script
    Reboot and it's good to go.  Indestructible! Even survives desktop reconfig due to monitor unplugs, etc.
    (added -nowireframe later because the lack of visible icon when dragging files was annoying. Take off if it bogs id down on slow connections.)