Missable - Very Important: After you kill the main boss Champion Victor turn his rare ergo in for the Frozen Feast Weapon. If you use the ergo for XP or the amulet you can not finish the mechanical vesselThere are 6 Cryptic Vessels but only 5 count for the achievement.
When you find a vessel bring it to Venigni in Hotel Krat and he will give you a a hint to the treasure.
*** Spoiler - click to reveal ***After the second black rabbit brotherhood fight you have the option to kill the hound. If you choose to do so you will get his cryptic vessel. I did not on my first playthough and still got the achievement. I don't believe it counts. If you want his vessel kill him and loot his body.
Crafted Cryptic Vessel - Found in St. Frangelico Cathedral when you are going through some wooden platforming. When you get to the top you can push a giant pot of fire to the bottom clearing the tar and revealing a secret path.
Solution: - Go to Elsyion bridge where you fought your first stalker. Throw an object at the body hanging from the bridge. A second clue will drop. Go to the house at Elsyion Boulevard walk onto the balcony and its the first door on the right.
Jeweled Cryptic Vessel - Found in Estella Opera House, down a long hallway with enemies. At the end there is a ladder you can climb down. Instead climb the smaller ladder up in the same room to find the chest.
Solution - Go to the spot shown on the clue and use the "check ground" emote.
Old Cryptic Vessel - After the green monster boss fast travel Barren Swamp Bridge and talk to Hugo. He will give it to you.
Solution - Go to the Path of the pilgrim and go to the house shown in the clue. Talk to the man outside he will give you a key to go in the secret house.
Rusty Cryptic Vessel - Sold from the Butler at Hotel Krat behind the desk. You have to have the shop fully upgraded for it to show up.
Solution - Fast travel to Barren Swamp Bridge where you met Hugo. The path is now open. Go to the end of the cave for the treasure. Come prepared A LOT of tough enemies.
Mechanical Cryptic Vessel - Found late in the story at Arche Abbey Broken Rift in a chest not far off the main path.
Solution - Fast travel to the abandoned apartment go out the upper balcony and use the photo to find the vendor. The vendor will not talk to you unless you have the Frozen Feast weapon equipped.
0:19 Crafted Cryptic Vessel
2:52 Jeweled Cryptic Vessel
5:03 Old Cryptic Vessel
8:54 Rusty Cryptic Vessel
10:58 Mechanical Crpyitc Vessel