Can you clarify what this is - a life-size Rubies Vader mannequin, or a mannequin on which one of the supreme Rubies costumes is displayed?
All manufactured clothing and costumes are required to have tags about washing on them. So if it's a Rubies costume, it should have care instructions on it. I'm pretty sure the Rubies supreme soft parts were all machine washable.
If it's a prop - which a pre-fab mannequin would be - there's no care tag requirement. If that's what you're talking about here, I would seriously recommend surface cleaning only.
My 2 cents.
Rick