There are 2 things you can do to dim down a Vertigo:
1. Replace the lamps with LL-200 bulbs, available at modt DJ stores. It's 200 watts rather than 300, so rather than having an output of 600 watts, the light will only have 400 watts.
2. Take one bulb out. This will reduce the effect to 15 beams and lower the output to 300 watts. The light doesn't look as good with one bulb, but it WILL reduce the output considerably.
Hope this helps!