It's fun for me ^-^

Hi there, glad to hear that you've been experimenting with the deck! I spent a long time perfecting this list, and after trying out a lot of different options I firmly believe that Eilyn's Choice is the best card for this market slot.
One of the most useful things to have in market cards is niche useability. Although Vanquish kills all the same units as Eilyn's Choice, it's much worse when you're facing down the lethal Cykalis, the Burning Sand that's about to kill you (played in Praxis Pledge, a tier 1 deck atm). It's also worse against Vicious Highwayman, Eclipse Dragon, Ghodan, Undefeated, as well as other less-played units. Secondly, it gives us a much-needed answer to cards like Grasping at Shadows, Flame Blast, Obliterate, Maul, and more. Although the extra power cost is certainly relevant, the added utility more than makes up for it. The fact that you can combo Eilyn's Choice with Wisdom of the Elders and Svetya's Sanctum makes it even better.
As for Permafrost vs Vanquish in the maindeck, I'm completely convinced that Permafrost is miles better. Not only does it cost 50% less, it also answers early-game threats like Teacher of Humility, Argenport Instigator, Blackhall Warleader, etc... while answering larger threats like Vara, Vengeance Seeker and Champion of Chaos almost as well. It even has added utility against cards like Rost, the Walking Glacier and Inquisitor Makto. I'm playing 2 copies of Pristine Light at the moment because I think it's simply a better version of Vanquish, and I would be wary of overloading on this kind of removal that only kills large units, as the most threatening units tend to be the 2-3 drops that'll hit you for 6-12 damage over the course of the game, not the big slow durdly vara.
Hope my explanation made sense!
Erik9099