Assuming the server is using the LightDM display manager and no users are logged in:
Run from server:
sudo /usr/bin/x11vnc -auth /var/run/lightdm/root/:0 -display :0
Run from client:
ssh -f -L 5900:localhost:5900 SERVERHOSTNAME -N -p 22 vinagre 127.0.0.1:5900