Why didn't Darth Vader tell Sidious that Obi-Wan disappeared instead of dying?

Question

After the events of A New Hope, Sidious believed Obi-Wan was dead. Vader apparently never told Sidious that Obi-Wan disappeared instead of being sliced in two on the original Death Star.

From the 1983 novelization of Return of the Jedi by James Kahn,

“Tell me, young Skywalker,” the Emperor said when he saw Luke’s first struggle had taken its course. “Who has been involved in your training until now?” The smile was thin, open-mouthed, hollow.

Luke was silent. He would reveal nothing.

“Oh, I know it was Obi-Wan Kenobi at first,” the wicked ruler continued, rubbing his fingers together as if trying to remember. Then pausing, his lips creased into a sneer. “Of course, we are familiar with the talent Obi-Wan Kenobi had, when it came to training Jedi.” He nodded politely in Vader’s direction, indicating Obi-Wan’s previous star pupil. Vader stood without responding, without moving.

Luke tensed with fury at the Emperor’s defamation of Ben — though, of course, to the Emperor it was praise. And he bridled even more, knowing the Emperor was so nearly right. He tried to bring his anger under control, though, for it seemed to please the malevolent dictator greatly.

Palpatine noted the emotions on Luke’s face and chuckled. “So, in your early training you have followed your father’s path, it would seem. But alas, Obi-Wan is now dead, I believe; his elder student, here, saw to that—” Again, he made a hand motion toward Vader. “So tell me, young Skywalker—who continued your training?”

And from the script for The Empire Strikes Back:

Emperor: We have a new enemy - Luke Skywalker.

Vader: Yes, my master.

Emperor: He could destroy us.

Vader: He’s just a boy. Obi-Wan can no longer help him.

Vader would say the Obi-Wan can no longer help Luke because he probably believes Obi-Wan is dead. Yet, he should not assume his previous master was dead because of what Obi-Wan told him a moment before his "death".

Obi-Wan: Strike me down and I shall become more powerful than you can possibly imagine.

For somebody to disappear was a trick Vader had never seen before. To the best of my knowledge, it was a trick nobody in the galaxy had ever seen before since Obi-Wan was the first to will his body into becoming one with the Force. (Qui-Gonn was a Force Ghost, but his body did not disappear as Obi-Wan's did. And according to Lucas, "We never see the ghost of Qui-Gon; he's not that accomplished. He's able to retain his personality, but he's not able to become a corporal ghost.") Nor was there anything in the Jedi teachings about Force Ghosts. It would puzzle, perhaps even unnerve, Vader.

Did Vader deliberately hide this from his master? We know Sidious could so easily extract information from the minds around him, so hiding such a crucial piece of knowledge would be difficult.

What exactly did he tell Sidious?

I am looking for answers from G-canon or T-canon, not lower canon levels.

Answer 1

There's a few points that play into this

1. Vader doesn't tell Sidious everything (and vice versa). In fact, it's a major plot point that Vader has been wrestling with his feelings internally, and Sidious is caught off guard when Vader acts on them and kills his master.
2. As far as anyone but Luke and Yoda are concerned, Obi-Wan is dead. Nobody else sees him, except Anakin at the very end of ROTJ. There's no indication Sith know how to Force ghost, and there's no evidence of any Jedi ghosts being sensed by Sith.
3. Vader seems to believe he's dead. That he disappeared is irrelevant. Vader says this in ESB

   Don’t let yourself be destroyed, as Obi-Wan did.

4. Even if Vader had mentioned it, what exactly would you do? The guy disappeared. The Sith didn't believe in life after death

   We shall see. There is no life after death. Only nothingness awaits you, Jedi. Your fear feeds our hunger for power. They will know you are here. We will tell them. You will die and be nothing.

Remember, Anakin only learned to Force Ghost in the last moments of his life.

Answer 2

Willed into the force or sliced in two, that doesn't change the fact that Obi-wan is dead.

According to canon sources, Darth Plagueis and Darth Sidius worked together to unlock the "secrets of immortality." Plagueis became an expert in midichlorian manipulation. In their research into immortality, they must have come across mentions of force ghosts and the like. In Legends, Sith were able to "force ghost" themselves, but due to the fact that true "force ghosts" had to release themselves to the Force (which darksiders refused to do), Sith Ghosts tended to be tied to specific objects or locations.

So, even if Darth Vader had told Sidius about Obi-wan disappearing, (at least with Legends information) Sidius would assume that Obi-wan had no more power; Sith holocrons made it clear that Sith ghosts would go insane and had little to no connection with the world after their death.

As far as Sidius was concerned, Obi-wan was done for and featured no more in Luke's life.