I think that if Vader is really trying to hide something like his knowledge of his son from the Emperor, he wouldn't go blabbing about it to all his crew. It's true that they are pretty isolated on the Executor, and they probably answer more to Vader than the Emperor, but contact is obviously possible. Either Piet or one of the men under him had to have intercepted the Emperor's call. In such situations, it would be easy for someone to blab it to Palpatine.