I have a couple of Vivitar 283's with missing sensors and found this information

http://www.hiviz.com/activities/guidebook/sensor.pdf

that shows how to hot wire a resistor accross the sensor contacts to get reduced power. I then discovered that Vivitar actually has a devide that plugs into the sensor socket to provide a calibrated version of this. VP-1 I think is the part number.

I'm assuming the 385 is a big cersion of the 285 which, I think uses the same sensor as the 283.