Assume that everything happened the same way in Chapter 6 of KOTET, but Arcann doesn't show up and intervene, so Vaylin winds up fighting the Outlander and Theron Shan in the ballroom. No outside interference, to the death.
She seemed solidly superior in Chapter 3, so I'm not sure why that would change so much now unless you believe the Outlander went through some significant amp in 3 chapters.
The Outlander continually improves throughout the story. You're forgetting that in this part, Arcann fights with Vaylin and has the upper hand or is at least equal to her yet if you choose to fight him the Outlander wins. The same would happen against his sister.