How to get yellowed satin back to ivory color?

I would try either ivory soap or woolite, using warm or cool water. Gently warp it in a white cloth (Lessens the strain of the vintage fabric) and gently submerge into the water. Let it soak for about 15 minutes the lift up and down a few times... gently squeeze out the water and place on dry towel to dry. when dry gently press with warm iron.

Do a search on cleaning and storing white satin garments. Then, make sure you store the dress in acid-free paper and boxes, never in plastic. Perhaps a dry cleaner that offers wedding gown preservation can help.

Is the fabric actual satin or a poly? Have you consulted with a cleaning pro?

Here are some sources of info for you:

oxy and dawn: https://fairychildheirlooms.wordpress.com/2012/04/16/whitening-a-yellowed-baptism-ensemble/

https://www.amazon.com/Retro-Clean-89865-Wash-1-Pound/dp/B00AXA4X2S/?tag=thrif06-20

https://www.google.com/search?q=napisan&rlz=1C1AOHY_enUS708US708&oq=napisan&aqs=chrome..69i57&sourceid=chrome&ie=UTF-8

Mix one part Dawn with two parts peroxide in your chosen container. Step 2: Then add 1–2 tablespoons of baking soda and water enough to soak.

1. see amazon for the product. If it is wasable, then Fill a clean tub or bucket with water (enough to submerge the fabric).
2. Add 3 tablespoons of Vintage Textile Soak per gallon of water. ...
3. Place the garment in the mixture and allow it to soak for 4 to 24 hours.

Remove the item and rinse it thoroughly with clean water.

http://www.howtocleanstuff.net/how-to-remove-yellowing-from-old-satin/

I've successfully removed yellowing from vintage fabric with Oxyclean. You may have to soak more than once and then air dry after rinsing it out.