When did Anakin/Vader encounter 3PO? maybe on bespin he saw him on Chewie's back, but I dont think vader was examing the droid to see if it was the one he made. TC-14 looks similar to 3PO, as does the other droid threepio encounters on bespin.
Same with Artoo. He wouldnt have even seen him at this point so when did Vader see Artoo in the OT? when he was trying to shoot down Luke's X-Wing in ANH? Im sure that was his concern. 'Oooh, I see a blue dome of an astromech droid...could that possibly be my artoo unit?' I dont buy it.
Uncle Owen. Similar info with 3PO, he has very little contact with Owen in ANH. He never is told or hears threepios name. Maybe he put it together when Luke mentiond the R2/Kenobi thing, but it wasnt like he was gonna say he recognized the droids at that point since he's trying to hide all that info about Anakin from Luke. And he did purposly tell luke to get the memory erased.
Obi-Wan, similar to uncle owen. Why would he tell Luke all that info that is obviously stuff that Luke isnt ready to hear yet, and wouldnt help their cause of saving Leia and the Rebellion anyway. (its like a need to know basis) He 'lies' to luke before we find out this info about the droids, but now that we know it it becomes more unacceptable, why?
preposterous fanboy justification? sorry, think your stretching just a bit much to discount reasonable explanations
-Darth Simon