No, I don't think Vader ever saw Threepio on the OT... On ANH, he MIGHT have seen C3PO while he was fighting with Obi-Wan ("this is our chance, come on"), but he wouldn't stop fighting and say hello to his droid. Besides, he didn't care anymore about him anyway...
Uncle Owen: Yes, it is a plot hole. Maybe Owen had his memory wiped out too. Or maybe he thought it was a different droid. Who cares anyway, Owen and Beru get toasted 20 minutes later anyway.
Obi-Wan: he didn't want to discuss them, maybe?
Of course, these are all huge plot holes because GL wanted to have the droids on the prequels so he could sell products with them, such as toys.