Typically the wiper motors will have a wire for a slow speed and a faster speed on them. It's really a matter of how slow you want it to go. If you purchase a PWM (speed controller) you can literally make the motor crawl. On the PWM there are 4 terminals, power in + & - and 2 terminals out to your motor + & -. Fairly straightforward to connect. We use some like these on our props and they work great. http://www.ebay.com/itm/291261719137?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT
Like Diabolik said, a PWM motor controller is the way to go for controlling speed. You can also slow it down by using less voltage. 5V usually provides a much slower speed than using 12V although it will also decrease the torque. Using a PWM controller will maintain the torque no matter what the speed is.
I don't think there's a way to directly do that but if you're good with mechanics it should be fairly easy to use a servo to adjust the pot of the PWM control. Controlling the servo position with VSA should be no problem. Of course it will take a little trial and error to get the servo positions you'll need for the program.