I think it could work both ways, as told in the Marvel comics, to bridge the years between Episode 4 and 5, aka "Vaders Search"
Or it could be that Vader soon after the Battle of Yavin found out (through interrogating rebel soldiers) who that mysterious pilot was, keeping the "info" for himself ...
in the end, the Emperor only recently came to the conclusion, that "the son of Skywalker" would become either a threat or his new apprentice ;-)