While Vader's suit does hinder him in many areas, he learns to adapt, and use it to his advantage (I think. I don't follow much of the EU, Cannon or Legends). He's now much stronger than most humanoids, and the suit does give him a lot of protection. In Ep V, Luke got a good blow on his shoulder, yet Vader didn't lose a limb. Considering the cutting power Lightsabers have, his suit must be very armoured. While it's very possible he's less capable than Anakin was at the time he took the name Vader, I still think he's more than capable of beating Ahsoka.