Obviously there needs to be SOME ambient light if there is to be a horizon from which to maintain orientation. The whole point of VFR is that you do not rely on instruments to remain right side up, or to navigate.
The only way to ensure there is ambient light is to take the phase of the moon in to account. Perhaps there should be a 7 day period per 28 days while the moon is less than one quarter full, when Night VFR is not considered possible.
P.S. And of course you would have to take in to account moonrise and moonset. Thankfully humans now have the ability to predict this sort of thing!
