He was a mystery for a common people, as it is stated that the Dark Council knew the truth about the Emperor's Wrath personality (Swtor Encyclopedia), about his life, prolonged with sith alchemy and so on. Only unanswered question back then was Scourge's motives. If Darth Vowrawn was studying Lord Scourge in order to neutralize him (same source), I doubt Emperor Vowrawn will keep a Sith betrayer by his side for long.