The original vacuum wiper and switch were on/off.
When on, the wiper blades move back and forth - somewhat continuously (but slow down/stop when the engine vacuum is low).
It sounds like your vacuum wiper "motor" needs to be
rebuilt -
http://www.wiperman.com/, or cleaned, or lubricated.
From an article by Bob Adler in October 2004. Vintage Truck:"Make Components Last Longer"
"Vacuum wipers aren't known for Herculean torque, but you can grow fond of the hissing and flip-flop sounds vacuum motors make. Periodically, squirt some motor oil into the wiper-motor air intake (a pinhole near the valve-tripping mechanism, not the rubber hose going to the intake manifold) as the wipers are operating. This will help to seal the moving paddle to optimize power.
http://1954advance-design.com/Web%20images/wiper-motor-lubrication.jpg
