Slayers stomp; both Arcann and Vaylin will fight anything similar to the Vongs for the first time in their lifes, and it's always been an insane game-changer; this is literally fighting against being from totally another galaxy, with half the things they've learned about combat being useless here. This is an insane thing even 1 on 1, but when you're gangbanged by a horde of Vongs?
If Arcann and Vaylin had the experience against Vongs like Kyp or Corran did, years of it, it could be viable they could win, especially with Lightning at their disposal, but they would still be hard pressed in an insane combat. Without that, Slayers 10/10.