And what would possess you to make such a claim?
Vader possesses twenty years more experience in the Force - specifically the Dark Side. He's spent those decades also actively hunting down renegades and Jedi Knights who managed to survive Order 66. His lightsaber prowess is not on the level of his RotS self, certainly not, but he is still formidable in that area. However, in Force prowess? It is a ridiculous assertion to think otherwise. He has more experience and much more control over the Force than what he had before, and he has likely mastered many more techniques than Anakin did. The obvious mental advantage is in Vader's court, as he applies tactics to his duels - and he is very much willing to use the Force when necessary (and he has Sidious's knack of manipulating his environment to his advantage).
It depends on the situation. Anakin will indeed curbstomp Vader if it ends up in a lightsaber duel. If he can make the distance and cross blades with Vader, his grace, agility, speed, and technique will give him a comfortable edge over Vader and let him win. However, should Vader decide to use the Force - he has an excellent chance of winning.
I am putting my money on Anakin, though. But not by a lot. Likewise, if Sidious has killed Mace, it won't really matter.