There is nothing that leads me to believe Kas'im is any threat. He couldn't beat Bane in a duel past a stalemate, and Vader is more skilled, faster, and just as technically skillful as Kas'im. If he can't even break Bane's defense there is no evidence to me he can survive an exchange with Vader, especially since he was being driven back in his duel with Bane. Vader would probably kill him as soon as they clash blades, Kas'im honestly sucks that bad
Raskta likewise isn't adding much even if she is more skilled than Ka'suck
Yeah. If Kas'im despite his technical ability was getting driven back by the raw power of a trainee Bane, Darth Vader, the guy who crushes cathedrals and matches the guy who one-shots AT-ATs and incinerates frigates, would break his arms. Rastka and Kas'im wouldn't make a very good team together given their fighting styles, so the numbers advantage wouldn't do much; Raskta herself was able to match RoT Bane, but Vader is by feats and accolades infinitely more powerful, and is himself a high level master of multiple forms and a legendary duelist. There's no reason to think he can't just let them get in each other's ways, quickly destroy Kas'im, and take care of Rastka.