If a refrigerator, dishwasher or other appliance has discolored vinyl trim, the appliance won't look clean. It's a simple matter to clean scuff marks and stains from white vinyl, but it's a different matter if the vinyl has yellowed. In that case, you can try bleach or vinegar, but success isn't guaranteed.
Dirt and scuff marks don't adhere well to vinyl, so you can get most of them off with a solution of 1 to 2 ounces dish soap per gallon of warm water. Use a non-scratch kitchen scrubber to loosen stubborn marks. If you need more cleaning power, use a solution of 1 part white vinegar to 1 part water. Avoid strong solvents such as mineral spirits as they can etch vinyl, which results in a dull finish.
Vinyl usually turns yellow because it's exposed to prolonged and frequent sunlight or because it's defective. A chemical reaction causes the yellowing so you may not be able to remove it. You may have some success with a solution of 1 part bleach to 2 parts water. If that doesn't work, try undiluted white vinegar. Allow the vinegar to remain on the vinyl until it dries, then wash it off with clean water.