I really wasn't keeping track of your discussion with Nephthys on this matter, but your implications that Sidious has no knowledge of Vitiate is facetious in nature, correct? Setting aside Palpatine's unparalleled knowledge of the Force and the fact that Vitiate's Sith empire isn't likely to be misplaced in the annals of history, we know that The Book of Sith features detailed accounts courtesy of Darth Malgus, a prominent Sith in Vitiate's empire, and these accounts found themselves in Palpatine's possession. The idea that Palpatine is ignorant of Vitiate's existence is... peculiar at best.
The Book of Sith is what exactly? And I was talking about familiarity with Vitiate's techniques and force powers, which is speculation at best.