According to a piece by the New York Times printed in The Spokesman-Review of May 25, 1980s (the same article also tells of a trilogy of trilogies; see this question):
The deliberately mythic names of the Star Wars characters took months to evolve. "Skywalker" was originally "Darklighter" and then "Starkill." "Darth Vader" was Lucas's careful blend of Deathwater and Darkfather. "Jedi" was chosen for its knightish echo of "Samurai," while "Obi-wan Kenobi" seems to Lucas both ancient and hypnotically phonetic.
The article is mostly an interview with George Lucas, so this information could have come directly from him. This is confirmed in Researching American Culture: A Guide for Student Anthropologists Edited (1982) by Conrad Phillip Kottak (this also appears to have been published elsewhere in 1980):
It is easy to note the phonetic resemblance of Darth Vader to "dark father." In a New York Times interview (May 18, 1980), just before the opening of The Empire Strikes Back, Lucas claimed that he chose "Darth Vader" because it sounded like both "dark father" and "deathwater".
An article in New Scientist (Dec 22-29, 1977) titled Science in folklore? Folklore in science? by Dr Alan Dundes says:
It may or may not be relevant that the archvillain's name is Darth Vader which strongly suggests death and father.
Literary Onomastics Studies, Volumes 5-7 (New York (State) State University College, Brockport, State University of New York College at Brockport, Conference on Literary Onomastics) (1978) says:
Darth Vader's name combines elements which reach to the very wellsprings of human emotion, for if I am not mistaken, Darth is a portmanteau or condensation of the words "dark," "earth," and "death." Its resonance with the word "dearth" is also notable.
Cinefantastique (Volumes 9-10) by Frederick S Clarke (1979) claims:
Darth Vader's name is a cross between the symbols "dark" and "death," and the German word vater [?], meaning father.
Science-fiction studies - Issues 21-24 by Indiana State University. Dept. of English (1980):
We realize now that Luke has been battling all along to kill his own father. The deliberate suggestion of "Death Father" in the name "Darth Vader" becomes transparently clear.