[sdiy] replacing resistor with vactrol??

[media at mail1.nai.net]

Let's say I'm designing a circuit that uses a resistor to control a timing
function (eg. the resistor determines the charging rate of a cap)  Are
there any rules of thumb to replace the resistor with a vactrol or LDR/LED
combo in order to control the resistor's value with a control voltage??
I'm unfamiliar with the specifications of such components, and looking how
I would choose the right part.

THANX!!