This is a story related, unmissable achievement...
As you would expect, the Phantaflask is found at the end of the second stage of the Motility Vaults section of the mansion. You will need to find the Vault key elsewhere in this level to get past the lock on the Vault door, opening the secret area beyond. The Phantaflask is in the Vault along with one Vitality Vial and one of the Newspaper Clippings (12/13). The red “Slimer-esque” Ghost will steal the Phantaflask, leading you on a quick chase through the Motility Vaults tunnels. Try not to get too winded during the chase, you’ll need your strength for the Boss fight that ensues.