There are plenty of explanations for the etymology of Vader and most character's names, but the question, I think, is how did Palpatine come up with it.
We don't know, but I would guess that he just chose it. Obviously not from the Dutch origin. Just something cool and intimidating, reminiscent of evil. He probably thought of it before hand.
How do you name a child? You just come up with a good name. This was the same, except instead of a baby, he was a Sith.