I got the Samisen and Orik's Shield (scenario B and 1 legendary completed) and the cards are in my collection. But how to play with them ???

I do not see them in my manage deck menu.

They don't show up in the "unclaimed" section when you are creating a deck after a scenario?

You only get it added to your Vault, not as a reward. So it may appear in later adventures as a random armor.

I have collected all 3 legendary level special cards and yes, all of them were in the vault and I met them in the video game locations later.
Not kept them, though because normal loot was better. But it seems to be working OK , technically.