No, you don't use an umlaut. An umlaut is merely a diacritical mark. The way the mark is interpreted depends entirely on the language. So no.
You have a scene early on in the books where Vaderwood is introduced and someone says "So how do you spell that?"
"It's spelled like Darth Vader, but pronouced like fatherhood with a v," or something.
Robert Parker does this all the time with his character Spenser.