ROTJ Vader is faster, more powerful, and more skilled at this point then he was when he last fought Kuro. While both of them have an environmental advantage none of them really have any sort of way to bring him down, they aren't skilled enough collectively to down him in a duel (Vader was inferior pre-ANH to his current stage), and TI obviously can't hold a candle to him in power or skill. Its only hypothetical that Felucia would give an amp to Kuro, so he can immediately dispatch her with the force and then just finish Ti.