Vader is actually quite powerful himself. Windu rivaled Yoda prior to his fight with Sidious and gave himself fully over to his vaapad which he has never done before making him, potentially, more powerful than Yoda in that exact moment.
Also Sidious was busy torturing Luke so he was caught off guard. Couple that those feats weren't in that body. He was able to do them in a younger more powerful body. Pretty much he returned far more powerful than ever. He's been dubbed emperor reborn. Sidious reborn would have destroyed Windu with no contest. Note Luke is even more powerful than Sidious reborn. So while Sidious was made "overpowered' by the EU it's nothing compared to how Luke has ended up.
Well.. I still don't think the Emperor reborn could beat Windu in a lightsabre duel, Force Battle Siidous wins absoloutely. But Windu is just below Luke in lightsaber skills on my list.