I am also having this issue. Did you eventually find it?
Viewing post in Finding My Steam Key
On the website, if you click your profile picture and select 'my library', you can see all of the games you've purchased from itch.io and below each of them is a download button. Select the download button for The Vale, and it will take you to a screen where there should be a button to claim your steam key. It wasn't there for me for a while, but it eventually showed up. I don't know if that's because falling squirrel did something, or if itch.io delayed the button showing up for me, or even some other reason.