I've always felt that Vader learns of Luke through Luke's own exploits.
During the years after ANH, it's likely that within the Alliance, Luke is a hero obviously. Luke's exploits I'm sure got back to Vader through Imperial Intelligence, or simple word of Mouth.
While maybe nobody else knew of Vader's last name, Vader would know Luke's name immediately (As would Palpatine, and maybe a handfull of others like Tarkin had he survived). The rest of the galaxy would just think of Luke as a hick kid who has proven himself on the field of battle.
The other story's taking place "behind the scenes", probably even behind Luke's closest friends own backs (Remember, Leia even seems to not understand why Luke leaves on Endor, and Han seems to be 100% clueless till Leia tells him that Luke's her brother).
Vader and the Emperor know though, as do Obi and Yoda... After Tarkin's death (And maybe even before, but he did know who Obi-Wan was, quite well, and he does refer to Vader as "friend" leading one to believe he knows his past) that seems to be it.