And it was by accident, too! “Vader” was used because of the English word “invader” because he invaded the rebel ship at the beginning of the first movie. At least that’s what I’ve heard.
I find the "invader" thing to be cool considering how other Sith have names are taken from evil concepts. Sidious = insidious (very fitting for Palpatine's character too). And the fact that Anakin did invade the Jedi temple with his clones.