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.