Vash expected that from you, it seems. He counters that with a Strong Dexterity.
You try to twirl your way out of the lock but Vash keeps his lightsaber solidly connected to yours, making any such maneuver awkward without leaving yourself exposed. Vash twists his lightsaber around and your lightsaber with it, and with a final twist he forces it from your grip. Your lightsaber sails over the handrails and down to the bottom of the stairs.
Vash raises his lightsaber to deliver a two-handed slash to your abdomen.
Rianna will respond by taking advantage of the fact that she's below him and ducks low so that his attack will simply pass over her, and them jump backwards down a few steps, using the Force to try and call her lightsaber back to her as she does so.
(do I need to name a move for this? If so, Normal Agility.)
It doesn't take Rianna long to creep her way into a metal pipe and nearly bang her head on it. This place is absolutely crowded with machinery. In the dark you cannot make most of it out.
Vash comes down the same way, but with his lightsaber off and his mind clouded to you. He slinks off away from the ladder, into the dark.
You can sense your way around the room. In the dim lighting, you can make out some sort of large piece of equipment that seems appropriate to ascend. You climb up a ladder and through some piping to reach the top of it. Whatever you have climbed on, there are still lots of pipes and hoses and deactivated control panels.
The ladder falls with a tremendous clang and your lightsaber glows vibrantly in the near darkness. You wander around whatever it is you are on, waving your saber around idly.
"No," comes Vash's voice over a PA system. It echoes in the big dark room and comes from all directions. "I want to understand my opponent, first. Why did you come here, Rianna? Why chase us?"
Rianna's going to keep sensing for where he could be, as well as for danger.
"Why?!" she responds angrily. "Are you seriously asking me that? Rand's manipulations made it so that the only way I could keep hold on my sanity was to leave the Jedi. Throw away any future I had there. I've nearly been killed by or because of him several times. And then I get a message saying that you've taken my family captive and dragged them into this when they have nothing to do with anything other than being related to me? I may not remember them, but do really you think that's something I could just walk away from and ignore?"
"Yes," Vash says. "You owe them nothing. They gave you away to the Order when you were a child and you had no say in it. And now you've rebelled against them. Why even care? All you continue to do is step into Rand's plots and encourage his behavior toward you. He didn't go through all this effort to recruit me."