Vinyl windows should be cleaned with a mild soap, such as baby shampoo, or a weak mixture of vinegar and water. Use a soft cloth, sponge or soft bristled brush, because anything else could scratch the glass or the vinyl frame.
For a soap-and-water cleaning solution, mix a small amount of dishwashing liquid with a gallon of warm water. Alternatively, mix 1 teaspoon of baby shampoo or 1.5 cups of white vinegar with a gallon of water. Never use products that contain strong organic solvents, bleach, grease remover, furniture polish or other harsh soap or detergent, as these can damage the vinyl and cause it to fade or discolor. Even mildly abrasive cleaners should be avoided, as these can also damage the frame and glass. Also avoid using a power washer on vinyl windows, because high pressure can damage the caulking and ruin the air-tight seal between the panes.
Vinyl is generally easy to clean, which is why it's best to start with soap, warm water and a soft cloth. These are normally sufficient for all but the dirtiest windows. Although commercial window cleaners are usually sufficient for removing spots and water stains, they often fail to produce clear, streak-free glass.