This occurred at our local fish restaurant. It is quite a large stain so when I got home I washed the jacket twice by hand in tepid water and wool wash and rinsed in cold water. Hung to dry but the stain is still there. The care tag says the shell is 100% nylon, 90%grey duck down, 10% small feathers, lining 100% nylon. Down-proof fabric 100% polyester. (A little confusing). Washing instructions are for cold hand wash only non chlorine-bleach when needed.
I checked with the restaurant and was told the viniagrette is made of blended vegetable oil, vinegar, honey and grainy mustard.
I would very much appreciate it if you can guide me in what to try next as if is a favourite jacket. (Murphy’s law). Thank you in anticipation and for your wonderful service.