TIMING NOT ADVANCING WHEN VACUUM IS APPLIED TO THE DISTRIBUTOR VACUMM CAN

Does the mechanical advance work properly, as in advancing smoothly from it's idle degrees to it's all-in degrees?That is to say;does the timing light show it advancing smoothly with no dropped sparks or jumping backwards from time to time? This type of monkey business speaks of reversed polarity to the pick-up. I don't know how that would play out with the Vcan.
But as said above; if the plate is rotating around it's anchor-stud, then it has to advance the spark, unless the oil-pump driveshaft is simultaneously climbing up the cam drive gear. That, IMO, is extremely unlikely.