Darth Vader remembers R2 D2

Why doesn't Darth Vader recognize C-3PO?

Droids are mass produced. Even C3PO, built by Anakin Skywalker, has a generic cover over its body. There are several other nearly identical droids throughout the series that prove that C3PO is not aesthetically unique.

Outwardly, C3PO is not clear. He has the same body as other protocol droids. You can't expect Vader to remember him because of his looks. The Sith Lord has likely had dealings with many such droids in the decades since he last saw C3PO.

I wouldn't recognize my first car 20 years later, even though I've seen MANY like this since then.

Now think of the name of C3PO, which may very well be unique: Remember that Anakin has experienced extremely traumatic events in his life, not least the transformation into a hideous, young, murderous, mutilated, walking corpse of a Sith- Lords. It's pretty easy to assume that there are things that Vader just forgot, either through choice or through unconscious oppression or through the trauma of being cut up, set on fire, and left for dead.
Remember, even Obi Wan said when Anakin became Darth Vader, Anakin himself was destroyed.

In the end, it's all speculation. The real answer is, when the original trilogy was written and filmed, Anakin Skywalker wasn't the creator of C3PO. No thought was given to who Vader would recognize as there was no need for Vader to recognize the droid at all. Anakin as C3PO's dad is just another thing got jammed into the Star Wars myth by a man who wanted to create some symmetry with his story without caring how it worked with the original trilogy because he himself is aware that he can change the originals at any time he wants.

Petersaber

Is C-3PO ever called "C-3PO" before Darth Vader? I don't remember it was.

Ghoti and chips

This should be the answer, but oh well.

vynsane

If I saw a baby blue '78 Chevy Malibu I would definitely have to take a second look at the driver's side corner of the front bumper to see if it is bent in, like my first car did after a fateful brush with one the case was lamppost in a McDonald's parking lot. Its presence would surely suggest that I had stumbled upon my first car, alive and well, which prompted further investigation.