Look, it would be a very close fight between Darth Vader and Darth Bane. Darth Bane has his orbalisk which protect him and heal him almost instantly and also make him stronger in the darkside of the force. But the orbalisk can be killed by force lightning or anykind of electricution as we see in Darth Bane Rule of Two. Since Darth Bane is aware of this problem though it would be very hard for Vader to pull off a force lightning attack without Bane catching it with his lightsaber and deflecting it. Bane is also a master with his lightsaber and after his near defeat by his former master Kas'im you know he would have studied all the lightsaber styles because that's what Bane does. He seeks to find all knowledge of the darkside and darkside arts no matter if it kills him in the process. He is also a quick learner and brilliant tactician and is a true Sith Lord.
Now for Vader. Darth Vader is powerful and it is true he is exceptional with his lightsaber skills but what no one seems to think about or realize is that he is not nearly as powerful as Darth Vader as he was as Anakin Skywalker. Why you ask, I'll tell you. We all know about midichlorians. Well when you start to lose pieces of your body you lose midichlorians. It's ok to lose something like a hand like Luke Skywalker, but Anakin lost his arms and his legs. Because of this he became less powerful in the force though he was still very powerful. If you have read Legacy of the Force books you know that Darth Vader could never be a true Sith Lord. Now in Darth Vaders defense, just because you are not as strong in the force does not mean you cannot use the force better than someone who is stronger in the force. It depends on the individual. All being stronger in the force means is that when you use the force it takes less of a toll on your body physically. Take Kyp Durron for example. In one of the New Jedi Order books Luke uses the force so much he passes out for several hours afterward. Kyp Durron has to do the same thing Luke did later on but he knows he is stronger in the force and after he does it he just has a bad headache. But even though Kyp Durron is stronger in the force Luke skywalker still defeats him when they are sparring because he is more knowledgable in the force than Kyp and is a master with his lightsaber. So it depends on the person and their knowledge of the force. But on the downside even though Darth Sidious was Vaders master, Sidious would surely have not taught Vader to much so he could still impose his will on him and keep him his servant. Just like in the movies when Vader wants Luke to help him defeat Sidious. Why didn't he do it himself? Because he didn't have enough knowledge and was not as powerful as everyone thinks.
So sorry Darth Vader fans myself include but Bane wins hands down. Would be a good fight though.