In Lords of the Sith:
“You’ll have to do this yourself, my friend,” the Emperor said to Vader. “I can’t be seen using the Force before so many witnesses.”
Vader openly uses his Force ability in front of people. E.g. in Star Wars.
Lords of the Sith is set 5 years after Episode III: Revenge of the Sith, so Palpatine's rule was secure.
The few remaining Jedi knew that he was a Sith, so it was hardly a secret to his main enemies. What was he concerned about in this scene?