While one could argue that since a carpet steam cleaner removes the water and dirt by suction, why should you bother vacuuming first. Though that seems to make sense, you shouldn't eliminate this important step. Vacuuming before you steam clean will remove loose dirt and sand from your carpet, so you won't be creating a nasty sludge that just makes the rug harder to clean. It also lifts the fibers so cleaning can go deeper. Consider it a pretreatment.