This page of our guide to Psychonauts 2 lists the Memory Vaults available in the first level - Loboto's Labyrinth.
Two Memory Vaults can be found in this level.
Enhancement Under the Sea
Location: Central Office.
To reach this secret you must destroy the boxes blocking your path. They're located on a metal shelf, on the left wall.
The Memory Vault can be found in a small room. It'll start running away from you. You have to hit it several times to calm it down, and then add it to your collection.
Psychonauts Ruin Everything
Location: Dental Void.
This secret can be found only when revisiting the stage. First you must acquire a skill - Mental Connection.
To reach this secret you need to jump across thoughts and reach a separate platform. Then lunge at the secret while it runs away from you and add it to your equipment.