Now, according to Lucas, Sidious > all.
But how could Sidious possibly defeat Vitiate?
Actually how can Vitiate possibly be defeated? My best guess would be that some millenia old Rakata mounting a Celestial wielding a lightsaber crafted by Masamune with the blessing of the Istari and with a machine gun for his left leg could have a chance to scratch the real Vitiate.
Sidious can eat a planet, create massive Force Storm all by himself.
The Emperor never showed greater feats than Nihilus.