To configure the computer to be able to start the X server without a monitor (making it able to render or to be connected with VNC) you need to make the xconfig so it doesn't try to negotiate (autodetect) the monitor and use the monitor value you give it.

To do so:

1. Connect a monitor to that computer and reboot.
2. Once the graphics have started run the nvidia-settings program.
3. Select the connected monitor.
4. Click on Acquire edid.


5. Now you have created a edid file (extended display identification data) for the X server to use instead of autodetecting the monitor. 
6. As the root user edit the /etc/X11/xorg.conf file to tell it to use the edid file.

My xorg.conf looks like this:

Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce GTX 960"
EndSection

So you'll need to add the sections telling it to use an specific output and the edid file you just created, it should look something like this:
Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce GTX 960"
Option         "ConnectedMonitor" "DFP-0"
Option "CustomEDID" "DFP-0:/PATH_TO_MY_EDID_FILE"
Option "IgnoreEDID" "false"
Option "UseEDID" "true"
EndSection

7. Reboot the computer and remove the monitor.