Use a spoon or dull knife to scrape away as much spilled dressing as possible. Don't rub or smear the dressing; just lift it away from the upholstery.
Sprinkle baking soda or cornstarch over the stain, covering it completely. Let the powder sit for 15 minutes to absorb the oil.
Vacuum away the baking powder or cornstarch.
Sponge dry-cleaning solvent onto the stain with a clean white cloth. Blot gently until the solvent is absorbed. You may need to repeat this step several times to remove the stain completely.