Comment 11 for bug 1741027

Here is an ugly 3 steps workaround:

-1) Edit the org.gnome.Vino schema to restore the missing "enabled" parameter (copying from 16.04)

sudo nano /usr/share/glib-2.0/schemas/org.gnome.Vino.gschema.xml

Add this key:
    <key name='enabled' type='b'>
      <summary>Enable remote access to the desktop</summary>
      <description>
        If true, allows remote access to the desktop via the RFB
        protocol. Users on remote machines may then connect to the
        desktop using a VNC viewer.
      </description>
      <default>false</default>
    </key>

-2) Compile the schemas for Gnome:

sudo glib-compile-schemas /usr/share/glib-2.0/schemas

-3) Now the screen sharing panel in unity-control-center works... but this is not enough to get vino running! So you need to add in the programs at session start: Vino-server with the following command line:

/usr/lib/vino/vino-server

Now you can VNC your 18.04-Unity!