From what I understand, lowering the voltage on a bulb will change the color temp of the light.
I do not know what that will do for VC paper. It may throw off the grades, so a #2 may print differently at full vs. reduced voltage.
Since they do not make photo bulbs lower than 75 watts (that I know of), another option is to use a regular WHITE incandescent bulb of a lower wattage, say 40 watts or even 25 watts.
I say this is not the best option because a regular bulb may not have an even coating of white to evenly diffuse the light. You also have to watch where the printing on the bulb is, vs the light path. If the printing is on the top end, and that is the path of light to the lens, you need to remove the printing from the bulb. I was considering this option myself, since a 75w bulb in my Durst M600 gives me a somewhat short exposure time of about 10 sec.