To use a MacBook (or MacBook Pro for that matter) with the video outputting to only the primary display and all the VRAM dedicated to that external display you need to:
1. With your MacBook on, ensure that the display is plugged in and working, you are connected to power and have a USB keyboard and mouse attached (or have enabled bluetooth devices to wake the computer). then,
2. Close the lid, wait for the MacBook to sleep.
3. Tap a key or click a mouse button to wake the system up - the external display will now have all available VRAM dedicated to it.