Add a dark blue gradient to the track above your video and change the compositing mode to Multiply (Mask) on the gradient track. Make the top of the gradient really dark blue and the bottom slightly lighter blue so that the bright sky is compensated for more than the ground. Set the color blue to taste. Also desaturate with the Color Corrector because colors are less pronounced at night.
If you've owned one of the previous versions of Vegas (I can't remember if it was V7 or V6) then you may have Magic Bullet Movie Looks HD which includes a preset called "Night Time" which is intended as a day-to-night simulation. Works pretty well.
If you are getting ready to shoot new footage for this sequence, then be sure to crank up your ND, or even add more physical ND on top of your lenses to obtain better dynamic range, so that the sunlight looks less harsh (this might help sell your moonlight)