i usually flush first with nothing but cold water. afterward you can clean in the normal way.
Whatever is left is usually discoloration on wool, should come straight out of polyprop.
If the customer has used vanish and scrubbed they have probably set the stain completely and when you apply a liquid dont be surprised to find the colour is bleached by the vanish.
afterwards you can sanitise.
Any yellowing may or may not respond to a reducing agent.
Works for me but test before you commit yourself.