Droids are massed produced. Even C3PO, built by Anakin Skywalker, has a generic cover placed over his body. There are several other nearly identical droids throughout the series to offer proof that C3PO is not unique in an aesthetic sense.
Outwardly C3PO is not unique, He shares the same body as other protocol droids, you can't expect Vader to remember him based on looks. The Sith lord was likely to have dealt with many such droids in the decades since he last saw C3PO.
I wouldn't recognize my first car 20 years later, though I have seen MANY just like it since then.
Now for C3PO's name, which may very well be unique, remember that Anakin has had extremely traumatic events happen to him in his life, not the least of all was the transformation into a hideous, youngling murdering, mangled walking corpse of a sith lord.
It's quite easy to assume that there are things that Vader has just forgotten, either by choice or by subconscious repression, or by the trauma of being cut up, set on fire, and left for dead.
Remember, even Obi Wan said that 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 was not the creator of C3PO. There was no consideration given to who Vader would recognize, because there was no reason for Vader to recognize the droid, at all.
Anakin as C3PO's father is just another thing wedged into the Star Wars mythos by a man who wanted to create a certain symmetry to his story, without caring how it worked with the original trilogy, because he's well aware that he can change the originals any time he wants to.