Please do not use foil or plastic to cover on your vent fan or your stove. Often people who do this expect that it will keep their appliance clean, but in fact oil can build up underneath, as well as on top of the foil or cellophane, all of which presents a fire hazard. In addition, burning aluminum can cause invisible fumes that are hazardous if breathed in.
Recently the fire department responded to a kitchen grease fire, where a stove burner was left on with a pan on the stove, and these items contributed to smoky conditions and set off the local smoke detector. Fortunately it was noticed by a neighbor and responded to, so damage was minimal.