The Fluffkin Vault is inside of Deep Elderwood. First you may have to defeat a few monsters to get there.like burly brambles
The Vault is initially locked and unlocks when the story-line finishes. You have to be Master level on Hunter, Carpenter, and Woodcutter. You will receive a letter from Pino with information after fulfilling all the conditions.
Inside the Vault Edit
Inside the Fluffkin Vault, you will find x2 red treasure chests and x3 purple treasure chests.