I would, however, like to put forth a hypothetical:
According to Knights of the Old Republic, 4000 years before ANH, it was customary for the titel of Sith Lord to be pasted from one Sith to another through assassination. An apprentice always rises to defeat their master.
If they are successful, then the Sith have grown more powerful.
If they fail, then the Sith strengthen themselves by removing weak apprentices.
Assuming the same tradition was kept, Palpatine should have expected Vader to do something. When Luke pops up, bingo, Vader now has an apprentice he can take and Palpatine should see the writing on the wall.
So, we shouldn't be surprised if Palpatine knows exactly what is going through Vader's mind.
However, Vader should know the same traditions. He should know the Palpatine would expect him to attempt to surplant him. Again, when Luke is discovered, he should know that the Emperor knows.
Why would he attempt to deny it to the Emperor??
Its at that point that I sense my head start to swell, and I cannot think about it anymore lest my head explode all over my place of work.