Can't you disconnect the Scangage and see if the problem persists?
I don't own a Scangage but I very highly doubt that it is altering the VSS or any other signal. That would be unacceptable.
I tried using a spark plug wire inductive pickup to get a motor speed signal from my fuel injected engine but discovered that under certain conditions, such as deceleration, the spark is stopped thus making it a discontinuous signal and unacceptable for a cruise control. The VSS should not be discontinuous though.