> Anybody know what is the maxium frequency of modulation a Vactrol
> wil be able to respond to

This depends upon the specs of the vactrol (mainly the material of the
photo-sensitive layer as the LED is much faster than the photo resistor).
Some manufacturer use secret identifiers (e.g. Perkin Elmer material no. "0"
or "3"), other publish the chemical details (e.g. cadmium-sulfite). In
addition the max. frequency depends upon the "brightness bias point". In the
dark area the vactrols are much slower than in the bright area. It will take
several seconds (for some materials even minutes) until the max. resistance
without illumination is reached. The max. frequency in the bright area may
reach up to some hundred Hz or even some kHz (with decreasing resistance
change level).

Best wishes
Dieter Doepfer