and deserving of an answer.
Unfortunately I do not have one but, creating a shortcut (alias) to the Monitors system preferences control panel might be just as quick as turning the second monitor off.
Then you know that all the 512MB is going to the one monitor.
Thinking this through though, with both monitors turned on, the System Preferences/Displays will give you the option of having two monitors displaying video.
With one monitor turned off, does the same system preferences pane now NOT see a second monitor and remove the dual display function from the Display system preference.
It does on my iMac, via a thunderbolt connection, which would indicate that all the VRAM is available to one monitor if the system does not detect a second monitor.
I'm using OS X 10.8 with two large LCD monitors (Apple and Sony).
My 27" iMac came with 512 MB of VRAM.
If the secondary monitor is turned off is there still only available 256 MB of VRAM available for my primary monitor.
I suspect that's the case and that my secondary monitor would have to be detached for my primary monitor to take full advantage of the 512 MB, but... can someone confirm this?
Also just curious... is there any kind of hardware that can be simply switched that would act as if my secondary monitor is unattached if this is the case. This would certainly be easier than having to unplug it.