I want to use an old vacum cleaner motor for a yard display project . How can slow the moter to reduct the RPM on the output shaft. I would like to slow the moter instead of gearing down the speed

Something like this should work for you:

Wall Control,120 V,Ivory - Fan Speed Control - Controls - HVACR : Grainger Industrial Supply (http://www.grainger.com/Grainger/items/4C254)

Just be sure the amps of the motor does not exceed the rating of any speed control you do use.