Probably because he is igniting his lightsaber and trying to cut through it...
You may be able to reach him before he makes any significant headway in his cutting. The cable is thick, so it's not something he can undo in a swipe. It won't take long though.
Would Rianna be willing to make a spend on Dash?
Vash no longer seems interested in talking as he tries to maintain his footing and scoot backwards across the cable. He keeps one arm raised behind him to help with his balance while parrying you thrusts with one-handed swipes. He is not quite as balanced as he had hoped to be, so your thrusts manage to push him back faster than he is willing to walk on the slippery cable.
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Rianna: 2
Vash: 0
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You carry on like this for a stretch, driving Vash quickly along the cable, with the Dark Jedi frantically trying to keep his balance. He manages to get some space between you both and swipes down at your feet, trying to delay your advance.
It makes you nervous that Vash's lightsaber dips a little lower than you thought he might and scrapes against the cable, but no significant damage is caused to either your legs or the cable so you feel alright about it.
You touch down to regain your balance, just as Vash turns to sprint back up the cable.
Round draw; goes with attacker.
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Rianna: 2
Vash: 1
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Rianna to attack.
Vash stops his dash and turns back, raising his lightsaber in a powerful upward slash to intercept your own. There is an echo across the skies like thunder as your blades collide with incredible strength. Your blade rebounds and you have to take a step back to prevent falling off the cable, as Vash's Strong Strength just narrowly beats your own due to him being naturally stronger.
-
Rianna: 2
Vash: 2
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Vash tries to knock you off of the cable by following up his superior defense with a series of horizontal slashes at your midsection, coming from the right.
Vash apparently learned from his mistake earlier, when you managed to unbalance him by stretching out his attacks. This time, he keeps things curt and close, and the repeated power blows manage to drive you back. He delivers one final one that makes you lose your footing, and you slip off of the cable...
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Rianna: 2
Vash: 3
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... but you catch it with your arms just in time, as Vash goes running off up the cable again.
Rianna to attack once she's recovered herself.
There's an alternative there, but we'll keep going with this one!
Ahhhhhhhhhh Rianna passes her roll and flips over Vash, which, needless to say, greatly surprises him. He tries to drive her off with a powerful blow, but she blocks it as she comes down and drops low to try taking him off his feet (or taking off his feet). He is forced to backpedal and nearly loses his own balance.
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Rianna: 3
Vash: 3
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Instead of trying to stabilize himself, Vash hops up again and tries to kick Rianna directly in the face.
Vash proves to be a little faster here and kicks you directly in the face, causing you to stumble back instead of hop. The result is that both of you lose your footing and both of you fall off the cable, only just managing to cling on at the last moment.
Round draw, goes with attacker.
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Rianna: 3
Vash: 4
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Your next six, please.
"I hate you!" Vash yells. Instead of pulling himself up, he is shimmying toward you.