Would it really? There was, after all, at least one other protocol droid on Bespin. You may say that Threepio and that other droid looked quite different, but how was Vader suppose to know what C-3PO looked like after all those years? After all, from TPM and AOTC Threepio changed looks completely and the only reason Anakin probably figured out it was Threepio was because of the voice (I say this because of the lack of recognition on Anakin's face until after the droid speaks, though, of course, it could also be the lack of protocol droids on a backwater moisture farm on a backwater, desert planet). I read that as well.