Será que Darth Sidious e Vader descobriram como enganar a morte?

Isso é abordado em detalhes no artigo do Palpatine de Star Wars Wikia, começar a ler de " Palpatine Renascer "seção.

At an unknown point in time prior to his death at the Battle of Endor Palpatine had found a different way to cheat death by preserving his spirit after the death of his body, by the method of spirit transference. He arranged for a series of clones of himself to be created for his spirit to possess, in the event that he would perish. His primary supply of clones was kept on Byss, guarded by loyal Dark Jedi and immense genetically-altered guards and maintained by a trusted private physician. Unfortunately, the clones were heavily ravaged by the dark side and not sustained by the Force. Each body he took would age and deteriorate more quickly than the last. But the Dark Lord wasn't concerned; he had an endless supply of clones which he could use to rule the Empire.

After the destruction of the second Death Star, Palpatine's spirit was forced to journey in the maddening, bodiless existence of the void, eventually taking possession of the body of Emperor's Hand Jeng Droga. Palpatine called out to Sate Pestage, who rescued the broken body from Kaal and returned it to Byss. Though Droga went mad in the process, he was able to journey to Byss, where Palpatine was able to take possession of a clone of himself. There, on Byss, he would remain for years and would rebuild his strength and Empire. Resurrected, Palpatine planned to replace the Galactic Empire with a "Dark Empire", a universe-spanning magocracy ruled through the dark side of the Force alone, without the need for regional governors or technological domination.

A maior parte é originária de quadrinhos do Dark Empire

No que diz respeito a Vader, a resposta é sim e não. Vader, o Lorde Sith, não. Anakin Skywalker o Jedi fez, de certa forma, desde que ele conseguiu aprender a se comunicar como um Espírito da Força (como evidenciado por ele aparecer com Obi-Wan e Yoda para Luke no final do ROTJ).

Saindo do romance que leva seu nome, é seguro dizer que o segredo de Darth Plagueis morreu com ele e não foi redescoberto por seu aluno:

Plagueis had discovered a way to influence a lifeform's midi-chlorians to either kill an individual (as he did with King Veruna) or to prolong their life (as he did with himself). As far his ability to bring people back from the dead, he performed this feat several times in his experimentation with Darth Venamis.

Provavelmente não

The sooner the [Death Star] is completed, the sooner you [Vader] and I [Sidious] can devote ourselves to more pressing matters - matters only you and I can investigate and that have little to do with the Empire."

[...]

Save for Sidious, no sentient being in close to five thousand years had set foot in the [Sith shrine beneath the Imperial Palace]. The room's excavation and restoration had been carried out by machines under the supervision of 11-4D. Even Vader was unaware of the shrine's existence. But it was here that they would one day work together the way Sidious and Plagueis had to coax from the dark side its final secrets.

[...]

As powerful as the battle station might become, its real purpose was to serve as a tangible symbol and constant reminder of the power of the dark side, and to free Sidious from having to portray that part.

[...]

Attacks like the one directed against Tarkin's moon base and discoveries like the one on Murkhana were distractions to his ultimate goal of making certain that the Force could not strike back

^{Tarkin Chapter 9: "As Above, So Below"}

Tarkin é definido cinco anos depois de Revenge of the Sith , então, evidentemente, nenhuma pesquisa estava sendo feita durante esse tempo.

Desde que a primeira Estrela da Morte não estava não totalmente operacional até os eventos de Uma Nova Esperança as melhores estimativas que temos , a Estrela da Morte foi destruída apenas alguns dias após a destruição de Alderaan.