A good way to remove yellow stains from a vinyl floor is washing the floor with soapy water and a mop. For tougher stains, purchase chemical compounds from your local household store, and use the chemicals to clean up the stained area. A brush and household bleach are handy in the process.
Clear the stained area of furniture and other items. Use a brush to remove dirt from the area. Alternatively, vacuum the place thoroughly. Pour water in a small bucket, add soap, and stir to form froth. Dip a rag in the soapy water, and use circular motions to handmop the floor. Use clean water to rinse the area, and buff dry. Ensure that you clean yellowing stains from vinyl the moment they occur.
If the stain persists, add a small amount of household bleach, oxalic acid or isopropyl alcohol to 2 cups of water, and apply the solution to the stain using a clean cloth. For maximum effect, leave the solution on the stain for a few hours, or even overnight. Rinse the area with clean water and a fresh rag, and allow the cleaned area to dry completely.
Make sure that you wear protective clothing while working. Test the chemicals on a small section first. Shut off gas supply to prevent explosions, and keep household chemicals away from children and pets.