Remember that Vader's whole suit, particularly his, ah, helmet are quite shiny (although it depends what shot how shiny) therefore your image needs more hilites.
Also, hia front chest plate has lights and stuff on it that are coloured.

Adding on this, if you wanted to be a bit adventurous you might try some lighting similar to Super Star Wars: Darth Vader sprite sheet
I always felt that this sprite was really unique and interesting.

The helmet also comes across as just slightly skewed, the left (ours) side seems like it need an extra pixel or two in places to get rid of that problem on the top round part.  Also the lower legs are about half a head short. In the lines it kind of works, but it doesn't work at all with the semirealistic rendering. You might also want to try using a couple pixels around the eye/visor parts to create definition.

Anyway, I quite like this. Sorry for posting this after you said you were finished.