Vitiate did however prevent his original body from expiring via natural aging. He kept his original body young and stable throughout his span of existence.
Incorrect, he needed to constantly transfer his essence into hosts like Abeloth, because his bodies could not meet the demands of his awesome power. Neither could Abeloth's.
The midi-chlorians are smart, they know when they're housing unnatural power, or being compelled by a foreign will (Darth Plagueis).
The only one capable of true immortality was Plagueis, by virtue of willing his midi-chlorians to undergo fission to replaced aged cells with new ones. Plagueis did admit that his midi-chlorian count had increased, and that his wounds had begun to heal and that he felt younger. He applied his scientific understanding to create a wholly new arcane ability in the dark side - another unnatural ability like Force drain, except it was better for achieving infinite longevity in a single body;
"The manuscript also included his Plagueis' on the Mortis legend, in which section Plagueis claimed that creating the Chosen One—not to save the Jedi but to be an extension of his will—would be a trivial matter once he had mastered the art of manipulating the midi-chlorians. The surviving manuscript concluded with Plagueis's interpretation of the Sith'ari prophecy as a description of his achievements, even though he rejected the concept of prophecy."
This suggests that he intended to gain more midi-chlorians than Anakin, and the powers he would gain would allow him to use very advanced Force abilities not yet featured by any human Sith in the EU except Abeloth:
"Darth Plagueis sought to understand the Force in a purely scientific context, doing away with the trappings of mysticism he believed the Jedi had corseted it with. He accepted the classification of the Force's aspects into three categories: the aperion (equivalent to the Unifying Force), the anima (equivalent to the Living Force), and the pneuma (conscious thought as expressed in the Force). His focus was midi-chlorians, which he believed benefited from a uniquely strong connection to all three aspects and could be used to master life and death. Apart from the extension and creation of life through midi-chlorians, Plagueis sought to explore such extreme and unusual applications as the direct manipulation of space-time and continuity of consciousness after death (going as far as to discuss the essence transference technique). He traveled to the Valley of the Dark Lords on Korriban, seeking contact with long-dead Sith, but found none in the tomb of Hakagram Graush or on the throne of Sorzus Syn. As he boarded his ship to leave the planet, however, he had a vision of the admonishing spirit of Marka Ragnos. The long-dead Dark Lord challenged Plagueis' claim to the title, and railed against his plan to dismantle the traditions of Korriban, but offered no answers to any of the Muun's questions and inquiries."