Configuring Ubuntu 12.04, to use VNC at 1024×768 without a monitor (tested using TightVNC from Windows).

Edit /etc/X11/xorg.conf (it's OK if the file is new):

$ sudo nano /etc/X11/xorg.conf

Section "Device"
Identifier "VNC Device"
Driver "vesa"
EndSection

Section "Screen"
Identifier "VNC Screen"
Device "VNC Device"
Monitor "VNC Monitor"
SubSection "Display"
Modes "1024x768"
EndSubSection
EndSection

Section "Monitor"
Identifier "VNC Monitor"
HorizSync 30-70
VertRefresh 50-75
EndSection

Now edit /etc/default/grub

$ sudo nano /etc/default/grub

Add nomodeset to GRUB_CMDLINE_LINUX entry:

GRUB_CMDLINE_LINUX="nomodeset"

Save file and…

$ sudo update-grub

Unplug monitor from the Ubuntu machine if needed and…

$ sudo reboot now

Test if you can VNC without a monitor to your Ubuntu, from Windows, using TightVNC for example.

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